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.