(x-1) + x + (x+1) = 138, so 3x = 138, and x = 46. Then, you do (46 - 1) and (46 + 1), and the answer is 45, 46, 47.
P.S. I'm only in fifth grade, so I think that you should be able to do something that a fifth grader is doing... but don't feel bad because I'm already doing calculus ^^
