It's not terrible complicated (unless you want to calculate it via a orthagonal projection matrix and even then it's not too bad). If you're going for a 2:1 x:y screen ratio (which is the usual one)
SCREEN_Y = ((X + Y) / 2) - Z
SCREEN_X = (X - Y)
Although it depends on what co-ords system you're using, but here I'm assuming that the X axis heads down-right, the Y axis head down-left and the Z axis heads up.
SCREEN_Y = ((X + Y) / 2) - Z
SCREEN_X = (X - Y)
Although it depends on what co-ords system you're using, but here I'm assuming that the X axis heads down-right, the Y axis head down-left and the Z axis heads up.