Map: cp_powerhouse
Easiest way to do this would be to enter an empty server on the "Control Point" playlist. If needed, vote to change the map using ESC, then clicking call vote on the small box.
Once you are on Powerhouse, change to Engineer. Before capturing the middle control point, place a teleporter entrance right next to the mid point and put the exit right next to the enemy control point. Once the tele is active, capture the mid point, head through the teleport and capture the final point.