"If three people wish to sell does the forth person have to agree?"
This is a very common situation, and the answer can depend on the circumstances surrounding the inheritance. People who leave houses to their children generally intend all of them to own an equal share, and unless there was a special condition attached to the legacy - such as a parent expressly wishing the youngest child to have a guaranteed home until the age of majority, for example - all parties must agree to a sale before the property can be disposed of. If one person refuses to sell there's not much the others can do.
As far as assessments for some benefits go, it's often the case that a local authority will consider what's called a 'beneficial interest' in a property as an asset, and they will place a value on it. This happens when assessing the amount an authority will be prepared to pay for care home fees, for instance.
As always where legal matters are concerned, you should take advice from a solicitor - I'm not one, and you shouldn't rely on what I say, or what anyone else here says by way of advice.