Personal oppinion following:
Madara is a little stronger than each one respectivelly... maybe about 15% stronger...
He has their KG, but we haven't seen him use their full potential, neither the special characteristics of each: He does not have the Susano weapons, and we don't know if he can use all the abilities of the rinengan (example: the statue)...
So, I think that, if they are in the same team, Nagato & Itachi can take this...