constraining object to a triangle plane I've been looking all over the place and I can't seem to find the right constraint for this. I have a circle that needs to be constrained to a triangles area. 

Limit Distance Constraint doesn't seem to work because it limits it to a sphere which can go out of the bounds of the triangle.

Limit Location Constraint doesn't seem to work because if my triangle's vertices are (0,0,0),(2.5,5,0),(5,0,0) and I max out x at 5 and y at 5 the object can go to (4.9,4.9,0) which is not in the triangle.

Any help would be greatly appreciated.
Create a new mesh shaped in the way you require the object to be constrained - in your case a single triangular face - and add a Shrinkwrap constraint to your object set to Closest Surface Point with Target set to the mesh. This will constrain the location to the closest point on that mesh. This will work for any shape of constraint.

To demonstrate, here a Copy Location constraint has been added to Suzanne to move her to the location of an empty. In addition, the Shrinkwrap constraint has been added and targetted at the triangular mesh.



Moving the Empty causes Suzanne to follow, but only within the bounds of the triangle mesh :