Three channels, kick, snare and kit all have their direct outs sent into a separate mix channel with a 6:2 mixer added as an insert effect. The master out of the 6:2 line mixer goes into the input of the mix channel.
I've put each left, right output in it's own channel panned hard left and right. So now you have a dedicated channel to mix the volume of your three kit channels. Oh, note. Make sure you turn the width to zero, else you'll only get a mono mix.