A treflip/3 flip/360 flip is a 360 shuvit and a kickflip done at the same time.
Hold down the stick for your back foot, and move it around in a semi-circle (in the direction your players back is facing) as you do this, push your front foot stick to the side. The same side that you scooped your back foot. Release both sticks to stop the board rotating and to catch the trick.
Be careful not to land some kind of variation on the treflip in between, or careful not to fall off. Here is a video I did of me completing this achievement (in regular stance for all you non-skaters or regular stance skaters) with the controller graphic turned on so you can see how I am doing them: