If the average of three numbers is v if one number is z and another is y what is the remaining number ?

The formula for an average of three numbers is (x+y+z)/3.  So if V equals the average, then we find the formula
V = (x+y+z)/3
We are now looking for x.  We can divide each of the variables in the parentheses by 3, making the formula
V = x/3 + y/3 + z/3
So now bring the unwanted variables to the same side as V.
V - y/3 - z/3 = x/3
Now multiply both sides by 3 to find x.
3V - y - z = x

x = 3V - y - z