Because you are. At all times all 3 servers are being double teamed. It may look like cooperation because you are fighting both servers, but it’s 99% of the time not the case. While you’re fighting both servers, a group from server B is probably fighting both servers and a group from server C is simultaneously fighting both servers. Sometimes this is even on the same map! Each map has more than one front. If you’re defending, and both servers are attacking you…they may not even realize the other is there! An organized group with the capacity to cooperate with another server would take advantage of the third party attack rather than do all the work themselves, just sayin.
Even if there is only on front on your map, on other maps I’m sure there are different battles where one side feels double teamed. Fact is, all of us are being double teamed at all times…because we all have 2 enemies at any given point.
Anyone can complain and call cheat, but a good server will learn how to thrive off of a 3 faction battle and come out on top…with or without cross-server alliances.