Thank you for your replies, Laurent and Sylvain. I guess my original question was not quite clear. Let me rephrase it: can I triangulate a Plane_3 so I can get a triangular mesh which represents this Plane_3? In this case, all the triangles will be coplanar.
The reason for this is that I have to have triangular mesh to do some other stuff, Plane_3 won't work for me.
Anyway, if it's not trivial to do this, I may try to avoid Plane_3 at the first place.