Unfortunately if you need 3 kids and all that goes with them, the M6 GC may not be what you need, the RS6 Avant would have the space and power, you need to check both out with the kit you regularly need and try to fit it all in.
I did this with the LCI M5 which I have and then with the M6 GC CP which I now have on order.
Got a feeling the M6 will not be big enough though.