How to prove 3CLIQUE is decidable
In fact, NP is not the set of problems that are easy to verify but hard to solve. It is just the set of problems that are easy to verify. NPC, the set of NP-complete problems, is the set of problems that are easy to verify but seem hard to solve.

Your problem is in NP but it isn't NP-complete (unless P=NP), since it's also in P. That is, it can be solved in polynomial time. To prove this, you need to give a polynomial time algorithm that, given a graph $G=(V,E)$, determines whether it contains a clique of size 3. I trust that you are able to do it yourself.