Question asked many times. In short beta decay.

Day 1: Index A goes from 100 to 80, losing 20%.

3x leveraged index A goes from 100 to 40, losing 60%.

Day 2: Index A goes from 80 to 100, gaining 25%. Total return over 2 days, 0%.

3x leveraged index A gains 75%, going from 40 to 70. Total return over 2 days, -30%.