Pillaging

Does pillaging C3 nations require a paid membership?

no

Well when I try to attack another c3, I already real eased my slave, it says free members cannot expand their empire beyond 2 countries.

Then you must de-register your slave country before you attack another C3 to pillage.

That's what I did. It doesn't show up in my countries list either. But it still won't let me attack!

If you wait for the update (happens once every game month) it will allow you to do so. In fact by the time you read this message, you can probably attack another C3.