Calling a 3-bet is destined to be a losing proposition, yes. But your aim when you press CALL is to finish the hand with a stack size that is better than it would be if you just folded to the 3-bet.
i.e. If you open for 2.5x, then when you call a 3-bet, you want your average loss rate to be -250bb/100 or better. If it's -700bb/100 over a large sample, you're calling too much, or playing badly in 3-bet pots.
Here's a snippet from one of my databases and it shows that, overall, I could probably call even more 3-bets, as I only lost 1.11bb per hand in this sample:
I may well have a problem in the SB, however, as I was losing quite badly (-5.4bb per hand) when I called 3-bets in SBvBB over a tiny sample. If your number is -700 for BTN, you almost certainly have issues that need addressing. Make sure you use the EVbb/100 stat to remove some of the runbad/variance. Note that you really need a much bigger database than mine or yours to come to some firm conclusions. (My numbers are from 125,000 hands, which is nothing really).