While you are a little sketchy on facts, I'm going to assume that you hold title as tenants in common (frankly it does not matter) but thats a good jumping off point for the analysis.
The tenant (owner) who wants out can file an action in partition and force the sale of the property, but this is unnecessary and expensive. You can do it yourself if all agree. All you need to do is agree on a price for the property and the two remaining owners pay the third one third of that amount to buy him or her out. That person will execute quit claim deeds conveying his or her in equal shares to the two remaining. The two remaining will each then own half.
Once everyone agrees, see an attorney for the paperwork. I would suggest that everyone divide his fee equally. Its worth it, especially for the two who want to keep the house for over time the appreciation will more than cover it.