Parametric data function fitting

Hello, i'm looking for a solution to the following problem: I have four functions:

f(x,y,10); f(x,y,15); f(x,y,20); f(x,y,25);

Now i need to generate a function f(x,y,z) with 10<z<25 but i dont know exactly how to do. Can anyone give me a tip? Thanks