My point isn't that you can make a line of lots of guns to salvo. It's that they are so cheap (in every metric- most importantly time to get a gun in position) compared to tanks.
I'm thinking specifically about a really annoying game on remagen where our tanks were getting blasted for cheap by AT guns that couldn't be kept down, because even if you found them and killed them, the little fudger would have another gun up within 1 minute. I think they had 2 guys building guns. Even though we killed something stupid like 14 at guns in that game we couldn't leave spawn to help our guys on the bridge because we had to constantly wack-a-mole the next at-gun. Granted, this issue is as much a product of bad map as it is poor AT gun balance.
3 shots to hull to kill tank at any range. that bit I don't have a problem with, I was just commenting the actual number because the OP made it sound like it had significantly gone up- which it just hasn't - it's +1, or +4secs time to kill.
I would also contend, that making a heavy tank retreat and need to repair damage is a win for an AT gun- similar to a recon managing to waste a tank's time being hunted rather than letting them drive to front. It takes ages to repair a heavy tank and pushing them into a corner is incredibly helpful to your team.
I think the number of shots is fine, but the O/A advantages of AT guns should have a cost to failure that is fairer to the cost of failure of losing a tank (i.e. the tank sufferrs 3 players out of action, a fuel cost and a 5 minute drive to front, whereas the AT gun suffers a minimum of a player death/respawn cooldown).