Nothing - beyond professionalism. It is common for knowledge transfer to happen in this time, and any outstanding vacation days are usually taken at this time.
There's a huge cultural difference between US and Europe (rest of world?) when it comes to going to work after serving notice. It appears the employer/employee relationship is more adversarial, so much so that the employee is not trusted/expected to do the right thing after being fired (or after they quit) - the whole "immediately getting escorted out of the building by security" is unheard of in Europe.
If you reach that you just put in the notice, take whatever vacations you have and then put in sick leave.
But 3 months notice is used for, so the company can hire a replacement for you, and you can help train him, and transfer knowledge.
A lot of people reading this site work in companies that have huge teams, but most people in IT I know from EU (where I live) are one or two person IT teams. 3 or 4 person team is considered almost medium sized.
In such situations you need yo hand of your task responsibly.
And that the one thing that will get you burned, if you leave your previous employer hanging.
If they are at the "fuck it" point, what's going to force them to be useful and productive during the forced 3 month notice period?