Are you sure the relationship has broken down. Have you talked about it with them or are you just pissed at them because you are working 16 hour days and they are working 6? Before I spent a penny on a lawyer I'd spend some money on a six pack of beers, sit down with them and make sure you really are no longer a team (especially as you mention in one reply a partner being a friend?).
So then assuming it is all over, next thing to do is to question why. There is a lot on HN about how amazing multiple founders are (personally I'm more a ME kind of guy). The other two don't want a piece of this action anymore, why? I've met very few people who are lazy and lack commitment when the rewards at the other end are worthwhile - does 2 out of 3 of your team think the idea is going nowhere. If so take a deep breath and think about it (I am not saying they are right, but take a deep breath anyway).
Are you at a point where you all hate each other? If not why not just say I assume you guys want out, you are not working towards the goal etc, I want to continue. I am not a lawyer and have little legal knowledge, but if you and your business partners agree that they don't want to continue with the business as it stands, and they are happy for you to continue by yourself, and they sign a simple document you all agree on (doesn't have to be a non-compete, just that they agree), then in 5 years time when you are the next Facebook they aren't going to get far - and you'll be able to afford the lawyers to protect yourself.
On what I assume is currently a zero revenue startup, where you are about to lose a large amount of 6 months of work and start again, communicate with your partners for free before you spend any money on lawyers. If they do not say what you want to hear, then go and talk to a lawyer (a decent one).