...stupidly, stupidly, I said yes to three MAs (I know that's a nightmare for uni admin but I couldn't make up my mind at the time)

"Yes" meant emailing/sending a deposit/saying yes to a very persuasive tutor who wanted to know why I hadn't replied.

Anyway, now I've chosen one, I’m worried that I might be charged/sued by the other two. Which would serve me right but would be pretty horrific.
I guess the other option is to become an action hero and do all three at once.

Thanks for any advice

I doubt a university would seek publicity by suing students! However, i think it would seem likely they'll hold back your deposits. Lets hope those other lecturers aren't reviewing your work anytime soon ;)

I would sue the hell out of you!!! only joking, don't do all three at once, with that workload you would fail all three, just explain to the other two unis that you have changed your mind and do not want to start the course there. They can hardly force you to come in, they would prefer to have a student that wants to be there rather than one that is there only because they are worried about being sued!!

Yeh tell them that you have had a better offer. They are usually understanding.