How do I find the 3 numbers that make up a sum? Eg: X+y+z = a

Whoa, Jackieboy.  If you have 3 variables, you need 3 equations to solve for them uniquely.  As it is, all we have is one: That they sum to A.  Even if we fix X at 25, for example, for any given value of A (say, 100), then all we know is that Y + Z = 75.  That's not much help, since any value of Y is possible, as long as Z becomes 75 - Y.  That means we can have any value of Y, and Z is determined.  So the value of Y varies freely.  And remember that in the first step we fixed X at 25; who says that's the value?  It can also float freely. So the answer is that you actually cannot answer the question until there are 3 equations, if there are 3 variables.