actually you don't. If you deal a random hit, and each dmg type is equally likely, i.e. 1/3 atk, 1/3 mag, 1/3 true and if you have the choice of boosting mag/atk by x/4 each compared to boosting atk by 2x/4 and mag by 0, you'll deal the same damage in expectation:

case 1:

x/(4*3) + x/(4*3) + 2x/(4*3) = 4x/(4*3) = x

case 2:

2x/(4*3) + 0/3 + 2x/(4*3) = 4x/(4*3) = x

when your choice is x/4 on mag x/4 on atk but y/4 > 2x/4 on atk (or mag) you should do the latter since in expectation, your damage will be greater:

case 1:

x/(4*3) + x/(4*3) + 2x/(4*3) = 4x/(4*3) = x

case 2:

2y/(4*3) + 0/3 + 2y/(4*3) = 4y/(4*3) > 4x/(4*3) = x

Therefore random damage is an equally good type of damage as true, since it doesn't really matter what you boost, same as for true.