multiplayer coop

is it possible to make a multiplayer co-op shooter using unity3d

Same-screen co-op shooter: Yes.

Split-screen co-op shooter: Yes.

Network/Internet co-op shooter: Yes.

Not sure which type of shooter you are talking, but I'm referring to any; First Person, Side-scrolling, Third Person etc.

So in short, yes.

I think you can make almost any kind of computer game in Unity 3D

You can make anything with Unity... If you can script...