So here's the rub: if you've already built an MVP, but have only spoken with 3 prospects to date, you're kinda going about this wrong (IMO). You should probably have spoken to dozens of prospects before writing a single line of code. Probably multiple dozens, actually. You shouldn't be writing any code at all until you already have a pretty good idea that the problem you're trying to solve is real AND that customers have at least some interest in a solution like what you're proposing to create.

There's also no real point in running ads when all you have is an MVP. Ads are for demand generation once you have a really solid handle on the problem, the product, sales channels, etc.

My suggestion would be the following:

1. Stop writing code or doing any development work

2. Cut expenses to the bone by any means necessary so that you're "default alive" no matter what (for the foreseeable future anyway)

3. Get yourself a copy of The Four Steps to the Epiphany[1] by Steve Blank. Read it. Then read it again.

4. Go through the customer development process as outlined by Blank, and start talking to prospects.

5. Don't start writing code again until/unless you sync up to that point where the process has you starting to take an MVP out to show to prospects.

6. Pick up from there and follow the process.

Now an exception would be this: you talk about a mature market. IF there are already plenty of well established, mature, market proven solutions that do more or less the same thing as your proposed solution, then you can skip some of the validation stuff. I mean, you could guarantee that you're at least building "something people want" by just copying one or more of your competitors. BUT... if all you're doing is copying a competitor, you have to ask "why would a customer buy your solution instead of theirs?"

Ideally in that case, you have some vision or idea for a "thing" that distinguishes your solution from the others on the market. In that case, though, you still have to validate that the market players actually see your proposed solution as better than the other solutions OR you need to be "at least as good as the others" but significantly cheaper, or something.

In terms of addressing all of that, a couple of books I'd suggest reading include:

1. The Discipline of Market Leaders[2]

2. Differentiate or Die[3]

3. Zero to One[4]

4. Traction[5]

[1]: https://www.amazon.com/Four-Steps-Epiphany-Successful-Strate...

[2]: https://www.amazon.com/Discipline-Market-Leaders-Customers-D...

[3]: https://www.amazon.com/Differentiate-Die-Survival-Killer-Com...

[4]: https://www.amazon.com/Zero-One-Notes-Startups-Future/dp/080...

[5]: https://www.amazon.com/Traction-Startup-Achieve-Explosive-Cu...

A couple that might be worth considering:

https://www.amazon.com/22-Immutable-Laws-Branding/dp/0060007...

https://www.amazon.com/Positioning-Battle-Your-Mind-Annivers...

https://www.amazon.com/Differentiate-Die-Survival-Killer-Com...

https://www.amazon.com/What-Sticks-Advertising-Guarantee-Suc...

https://www.amazon.com/Made-Stick-Ideas-Survive-Others/dp/14...

This starts as a pet project which was an excuse to keep in touch, experiment with new technologies and most importantly have fun.

Given that, it sounds like you've already won, no matter what happens next.

As far as advice goes, I'll just leave a couple of book recommendations.

https://www.amazon.com/Discipline-Market-Leaders-Customers-D...

https://www.amazon.com/Differentiate-Die-Survival-Killer-Com...

https://www.amazon.com/Positioning-Battle-Your-Al-Ries/dp/00...

https://www.amazon.com/Its-that-Small-Fast-Slow/dp/006662053...

Classics:

https://www.amazon.com/Positioning-Battle-Your-Al-Ries/dp/00...

https://www.amazon.com/22-Immutable-Laws-Marketing-Violate/d...

Seth Godin is another author well worth reading:

https://www.amazon.com/Seth-Godin/e/B000AP9EH0

And a few others:

https://www.amazon.com/Crossing-Chasm-3rd-Disruptive-Mainstr...

https://www.amazon.com/Kellogg-Branding-Marketing-Faculty-Ma...

https://www.amazon.com/Differentiate-Die-Survival-Killer-Com...

https://www.amazon.com/Confessions-Advertising-Man-David-Ogi...

And, of course:

https://www.amazon.com/s?k=vaynerchuk

There are no magic pills though. The best thing is to read and learn from a lot of perspectives and then start to navigate the waters you happen to be sailing. No two markets or products are going to be the same. In fact, the same product will require a different based on where it is in the adoption/maturity cycle.

This reminds me of my favorite quote from my favorite business book, Differentiate or Die by Jack Trout, with emphasis on "Anything that others could claim just as well as you can, eliminate.":

The best way to really enter minds that hate complexity and confusion is to oversimplify your message. The lesson here is not to try to tell your entire story. Just focus on one powerful differentiating idea and drive it into the mind. That sudden hunch, that creative leap of the mind that "sees" in a flash how to solve a problem in a simple way, is something quite different from general intelligence. If there's any trick to finding that simple set of words, it's one of being ruthless about how you edit the story you want to tell. Anything that others could claim just as well as you can, eliminate. Anything that requires a complex analysis to prove, forget. Anything that doesn't fit with your customers' perceptions, avoid.

http://www.amazon.com/Differentiate-Die-Survival-Killer-Comp...

"Differentiate or Die"

Google this phrase or buy the book:

http://www.amazon.com/Differentiate-Die-Survival-Killer-Comp...

My favorite passage:

"The best way to really enter minds that hate complexity and confusion is to oversimplify your message. The lesson here is not to try to tell your entire story. Just focus on one powerful differentiating idea and drive it into the mind. That sudden hunch, that creative leap of the mind that "sees" in a flash how to solve a problem in a simple way, is something quite different from general intelligence. If there's any trick to finding that simple set of words, it's one of being ruthless about how you edit the story you want to tell. Anything that others could claim just as well as you can, eliminate. Anything that requires a complex analysis to prove, forget. Anything that doesn't fit with your customers' perceptions, avoid."

with particular emphasis on:

Just focus on one powerful differentiating idea and drive it into the mind.

Why should anyone choose you?

Similar to the message of one of my favorite marketing books:

http://www.amazon.com/Differentiate-Die-Survival-Killer-Comp...

My experience is that none of the self-help books ever did very much for me. This includes Steven Covey, Dale Carnegie, Tony Robbins, Zig Zigler, the Secret, etc., etc., etc. Some were entertaining, but that was it. I understand that these books do help many people who need their messages; I'm just not one of them. I prefer books that inspire me and tell me what to do.

My favorites?

How to Get Rich by Felix Dennis.

http://www.amazon.com/How-Get-Rich-Felix-Dennis/dp/009192166...

Differentiate or Die by Jack Trout

http://www.amazon.com/Differentiate-Die-Survival-Killer-Comp...

Art of the Start by Guy Kawasaki

http://www.amazon.com/Art-Start-Time-Tested-Battle-Hardened-...

The 22 Immutable Rules of Marketing by Al Ries and Jack Trout

http://www.amazon.com/22-Immutable-Laws-Marketing/dp/1861976...

Digitial Aboriginal by Mikela Tarlow and Philip Tarlow

http://www.amazon.com/Digital-Aboriginal-Direction-Instincti...

and, of course

Founders at Work by Jessica Livingston

http://www.amazon.com/Founders-Work-Stories-Startups-Early/d...

Hackers and Painters by Paul Graham

http://www.amazon.com/Hackers-Painters-Big-Ideas-Computer/dp...

Before you do anything else, go to paulgraham.com and read all of the essays! I don't want to sound like a shill or hero worshipper for pg, but, understand, these essays are the very best thing out there for anyone on this forum. I am here because of the essays, not the other way around.