kinetic energy = (1/2) (mass) (speed)² .
see that little ² there on the end ? that tells us that kinetic energy is proportional to the square of the speed.
if the speed of the moving object triples, then the moving object has 3² or nine times as much kinetic energy as it had before.
a better way to look at it is: if you want to triple the speed of a moving object, it's not enough to just give it 3 times as much kinetic energy as it has now. you have to give it 9 times as much as it has now.