Consider events A,B defined as: A = (at most 2 functioning) and B = (all 3 functioning). A and B are complements of each other. In other words, B means: not A. So P(A) + P(B) = 1.

P(all machines not working) is (1-1/3)(1-1/4)(1-1/5). Do you see why?

Edit: Originally I used X,Y in place of A,B not noticing that this choice of letters conflicted with the machine names.