It can be done in two steps ONLY !!
make two groups of 3 balls each and one group of 2 balls (i.e 3+3+2=8)
Step 1: Measure group of 3,3 on the weighing machine.
case 1. If two groups are equal then the defected(less weight) ball is in group of 2 balls.
Step 2: Measure those two balls, you will get the defected one. [2 steps only]
case 2. If one of the group of 3 balls is less heavy then that particular group contains the defected ball. divide this group into 3 groups of 1 ball each (i.e 1+1+1=3).
Step 2: weight any two balls ...If one of those two is less heavy then that is your answer.if both are equal then the third ball is answer. [2 steps only]