Since the 3 boxes are indistinguishable and so does the ball, so it doesn't matter at which box and at what position that we should to put the balls in, so we can find that:
[5] [*] [*]
[4] [1] [*]
[3] [2] [*] OR [2] [3] [*] OR [3] [*] [2] OR [2] [*] [3] (the position doesn't matter since the boxes are indistinguishable, so these are counted as 1 way)
[3] [1] [1] (so does this one)
[2] [2] [1] (so does this one)
So, there are 5 ways altogether. We do count the first 3 as the ways too because the question doesn't state whether the boxes has to be fill with at least one ball. It can be any ball in the boxes including no balls at all.
Indicator:
[*] : no balls, just an empty box
[ ] : box
