3 Vertices

When I do a cameraPick To Get Picked Triangle, I can obviously Get The 3 Vertices That Make Up That Triangle, What I can't figure out is, How to get the Whole Square. When looking top down on a Mesh, in wireframe. The Triangles Hypotenuse matches up with another Triangle. I want that Other Triangle 3 Vertices.

Make Sense?

Thanks in Advance for any help,
Eric