Time consistency

2 and 3 are incorrect because the time used for each verb is inconsistent.

The most correct sentence is future: “On se verra quand tu seras libre.“

Sometimes though, present can be used to express a future that is not that far: “On se voit quand tu es libre.“

Future with aller

A near future can also be expressed using aller with an infinitive form: “On va se voir.“

Yet, the suggestion is indeed incorrect: “On se verra quand tu vas être libre.” There is a time inconsistency but I fail to express the rule about it.

This syntax is mainly spoken and spoken language tend to bend or escape the rules. Sentences using this construction are often short and I can’t think of an example with another verb in the same sentence.