How about lowering the threshold for players switching from a high ENY country to a low ENY country? If you switch to country A and they get some 15 ENY later that night, you can switch to one of the other two countries.
Switching to a country that has equal ENY or higher can remain at some "1 switch/night" number, and switching to a country of significantly less ENY can take no more than 60min.
Using ENY or low/high numbered sides would result in the same thing, ENY is determined by the numbers of one team vs the numbers of the other teams. I do like the idea of using ENY as its an already established game mechanic for describing it though.
as for DaddyDrax, a -1 comment is not helpful, explain why.
The idea is that so when numbers a low a fight can be still be maintained. Also, as people log off as the sides can progressively be rebalanced as the eny will change enabling people to switch to the lower numbered side balancing game play.