1. You really did disappoint at the end of your internship. I've seen this happen. People get complacent. Take a good, hard look at what you've done and see if there's any truth to it or that someone could get that impression;
2. The company's situation changed. Funding may have fallen through. There may be a power shift in management. Who knows? or
3. They were lying to you the whole time.
I suspect (2) but, then again, we only have your side of it. I don't mean to sound harsh here. I know nothing about you.
It's time to look on the bright side: you got a three month internship out of this. If you can find someone from the company to get a reference from then that's valuable.
The US operates basically on "at will" employment. With very limited exceptions (eg the Americans With Disabilities Act) they can withdraw that offer and that employment at pretty much any time. If you can demonstrate damages and they can't justify it, you can sue but you'll need to show damages and honestly it's not worth going down that route even if there is a breach of contract, which is a somewhat separate issue.
If you had offers from other companies, go back and contact them. Tell them you'd originally planned to keep working for the company you were interning for as you were really excited about what you were working on and who you were working with but that offer fell through. This is a perfectly acceptable situation and one that won't tarnish you as a potential employee.
If the company was dishonest with you, either from the beginning or at the end, then what happened is a good thing. Better to discover this now than 1-2 years from now when they shaft you out of your options or the like. Integrity and honesty matters. Do not go work for a startup if you don't trust who you're working for. Reputation is everything.
Lastly, I'm glad (for your sake) that you didn't drag whoever this was into the public by naming and shaming them. As tempting as that might be, don't do it. You seriously undermine your ability to get any reference from someone working there and, honestly, this tends to make whoever is slinging mud look bad as well. Just move on.