recon124, the thing is, you dont need to think of it as a scales synergy between your nations. More like an overall nation synnergy. So for example if one of you has really good armies(say MA Man) the other one has really strong SCs and thugs(to teleport and raid around) then the other one has stealthy thugs so that you invade a person in 1-2 turns alltogether. And lets say the last person has good mages so he helps with forging of itmes and other stuff. That is a good synnergy.
Do not striclty think of it as Scales synnergy since scales are most likely going to be valid only for your own dominion, and the opponents are going to preach it out.