I am an entrepreneur based in San Francisco who has spent 3 years building a healthcare-related company. I have dropped $150K into the product. I have a few customers but no traction. I feel like giving up, but $150K is a hell of a lot of money to have put in.
Unfortunately, this is, by far, the norm, not the exception. Most startups have this outcome. I have advised numerous entrepreneurs, and had to make the very same decisions myself - more than once. It all comes down to market potential. Does the product solve a problem/need that the market demands. After 3 years, if your product is substantively developed, does it solve a problem/need - really, does it? If it doesn't, either pivot to get it there or abandon. If it does, why aren't customers knocking down doors to get it? It is likely due to sales/marketing not working effectively (bad value proposition, pricing, business model, poor sales people, etc.). Either way, the market is speaking loudly right now to you...change asap or abandon. If you would like to discuss your specifics or ask follow on questions, I would be happy to help further.