How to make a prism with different shapes at each end? I'm trying to make a triangular prism that becomes circular at the other end, but to no avail.

At this stage I have a circle with 3 vertices and another with 16, how would I connect these into a prism?
add 13 vertices to your triangle, select all with A and connect with LoopTools > Bridge (tools tab in T panel). Can't screenshot now.