Why were certain trade routes called the triangular trade routes?

If you buy from A and sell to B, then buy goods from B to sell back to A, then even a casual passerby can duplicate the trips and easily compete and undercut you.

Buy from A, sell to B, buy from B, sell to C, buy from C, sell to A is much more profitable because the total trips average all shorter (draw a diagram and see!!!) and much more tricky to compete with.