Wikipedia has an article about it here.
In short a 3-SAT problem is a 3-CNF satisfiability problem.
A SAT problem is to figure out whether there is an assignment of the booleans that make the expression true.
A 3-SAT specifically restricts the type of problem to 3-CNF expressions.
It is not clear to me what a 3-CNF problem would be, which could be anything involving 3-CNF expressions.