Maybe your founders simply never sought investment to begin with, but being underpaid for 3 years tells me it may be time to cut your losses and to start looking at this in terms of opportunity cost.
But oh no! the boat sunk! the statue is well protected, but its at the bottom of the ocean! You can see the statue, glimmering at the bottom of the ocean, but you can't reach it.
So you do what anyone would do, you try to retrieve it. You think to yourself "this statue is beautiful, and its worth so much money! If only i spend less than the statue is worth, then I've made money!"
You send little robots and divers and all manner of things to try and retrieve the statue. Months turn into years, but you try to get this statue. You can see it, its right there just out of reach!
And suddenly, you've spent OVER 500k to try and get this statue back. You've lost money trying to retrieve it. And its still there, at the bottom of the ocean. Glimmering and shiny, taunting you.
Basically, this is the position you're in. You've essentially been donating your potential earnings towards trying to retrieve this statue. It might be time to give up on the statue, and go find some other shiny thing.
If you are gaining significant non-monetary benefits (eg learning a lot from much smarter people, really interesting problems), there might be a reason to stay, but if you are being underpaid and there is no reason to expect it to change, I would get the hell out.
Unless you can expect something to change fast, you might be better off looking for new opportunities.
I would say leave if you can get a better job elsewhere