You have to reach 2 digit stats (hence double) in three stat categories (hence triple).
The stats I suggest are points, assists, and steals.
Set the reach-in foul to zero, so that the referee will let you steal the ball every time. Make sure you get 10+ steals, 10+ points, and 10+ assists with 1 player, and you'll get him a triple-double. You should use a point guard or any offensive player. You can also adjust the sliders to your advantage, and set the timer to longer halves if you have trouble (5-10 mins is enough).
Other stats that work are blocks and rebounds.