I have this 3D Triangle of which I want to get the interpolated normal of V3.
I know how to get the normal per triangle face, but I don't know how to get to interpolated normal.
I would calculate cross(v4-v1, v2-v1), but that would give me only the normal for the whole face.
Can you help?
I suspect you are looking for the vertex normal. As stated in the link, it is the normalized average of the surface normals of the faces that contain that vertex.