Comparing 3 values.

aarondc:

bool (x_is_biggest(int x, int y, int z)) {

return ((x > y) && (x > z));
}

Too many parantheses! Certainly you need to loose the cast to bool and change to a function definition (boolean
is the preferred type for Arduino code I think):

boolean x_is_biggest (int x, int y, int z) {
    return x > y && x > z ;
}

return doesn't take parentheses like if and while do, lots of people think it does, also && and || bind lower than comparisons.