to write a do while loop to solving a non linear equation

I have three non linear equations which are : f(x),g(f) and x(g). I have to solve for x iteratively first by guessing x, substituting it in function f,f in g and at last g in x.If x obtained in last function x(g) is different from x guessed then it has to be guessed again(i don't know the logic for guessing next x ) and has to be substituted again f(x) till the x obtained from x(g) is same as x guessed .Any idea how to do this ?