Yeah, the main focus is 3's but they do make balance decisions based on performance in BGs. Hungering Cold was changed because of BGs. This is what they said at the time, "We added the cast time to Hungering Cold for PvP reasons. It is one of the most powerful forms of crowd control in the game, especially in Battlegrounds, and yet was impossible to prevent."Battle Grounds aren't used for balance because the ability to win is decided on a goal. Flag caps and resource points are more valuable than killing the opposition. In theory, every BG is able to be won by not making a kill on a player.
It doesn't happen as often but they've said they keep their eyes on bgs.
I am curious when they first started coming out and saying that they balance around 3's though. From what I've read, a lot of people liked season 3 and season 7 for arena, and I'm sure these seasons had issues, but did they start balancing around 3s before, between, or after? If it's after both seasons then I find that pretty funny since that would mean that players liked arena a lot more when the devs didn't try so hard to balance pvp around 3s, the way they do now.