That is the only benefit to 2 sides, and I personally think it would be offset by one side constantly having higher numbers and facerolling the map.
There is no offset. The maps are currently, and commonly, being rolled by higher numbers, apparently. So that statement implies that 3 sides is not suffering that same problem.
The better tool to adjust that is ENY, and possibly Supply-ENY cranking up more and more painfully until numbers even out. When the high number side is reduced to flying DR.I with no bombs, and no troops, and 1/4 fuel against the other side with free ME-262, numbers will balance. Operant conditioning can work wonders. Of course, you have to have a reasonable switch timer so players can adjust to get out from under that. That would work with 2 or 3 sides. (Note: that won't occur just because one side is losing. It only kicks in with numbers imbalance.) That should be cranked up even with 3 sides. But you do have to allow them to switch so it can have it's intended effect.
Two sides would have at least two main advantages, IMHO.
1. It would have on average a shorter linear frontage thus increasing density along the front for any given number of players.
2. Any fight in the arena is equally accessible by all players. No ignored teams looking over at a fight they'd really like to get over to and join.
3-sides worked great when you had more players because it tends to spread the fights around on more frontage and that improves server performance. And with enough players, there are plenty of fights everywhere for everyone.
At this point in time, spreading players around is probably counter productive.
As far as the argument that it creates ebb and flow, I find that weak. I've played plenty of games of two-sided Battlefield that had amazing ebb and flow. I've seen some truly heroic comebacks.
Do two-sided NFL games never have ebb and flow?
Why don't they have 3-side NBA?
Unfortunately, even 3-sides wouldn't make baseball interesting.
Hmmm now 3-sided hockey is something I'd actually pay money to watch.
But yeah, maybe the next game.
It doesn't have to
BE two-sided. it should just have the ability to configure two-sided the way you can configure a 512x512 map or a 256x256 map depending on your current needs.