If i've understood your question correctly, we have:

x/3, x+1/5, and x+2/7

If thisis right then we can just simplify the expressions x+1/5 can go to x/4, and x+2/7 can go to x/5

therefore we want a number that is divisable by 3,4, and 5. The the easiest way to find with these number is to multiply them, 3*4 = 12

12*5 = 60.

So i think 60 is your answer, however i'm not sure that that was your question, it depends by what you mean by '3 divides x' etc.

Anyways hope this helped.