Trial 3 is the Air Trick Trial. The board and character selection does not really matter for the trial so just pick what you like. Prior to starting the event the game will run you through each task and give you a diagram of how to do each of them. In Trial 3 you need to do the following in a single run:
"Do tricks by twisting your body in the air." Just twist around a bit while airborne to do different tricks.
"Raise your front leg while in the air to do a front grab." While in the air and standing sideways, kick out your front foot and hold it for a second or two.