I know absolutely nothing about FIU or their program. However, I would not suggest accepting an un-funded offer, regardless of maybe having outside funding. I think you would be much better served waiting another year and improving all the areas of your application that you can (GRE scores, Statement of Purpose) and applying to more schools next year, ideally more than 3 if that is financially feasible for you.
I was turned down from both schools to which I applied last year, but applied to 7 this year and have received funded offers.
I hope you hear back from the third school and get accepted with funding!