To make a triangle:
MAKE OBJECT TRIANGLE Obj,Xa,Ya,Za,Xb,Yb,Zb,Xc,Yc,Zc
Pass a set of 3 coordinates and you have a triangle.
This might be a good start but really you should look at the other options for mesh manipulation - memblock meshes and vertex modification.
It depends on how you want to work with it, but you will probably have to get something a bit more powerful, personally I think that memblocks should be learned by everyone - your in a much better position to make a model editor once you know how memblock meshes work.