In FR, against a player that's truly unknown, I'd still have a hard time laying down KK. I think in this spot the onus rests on showing that his range is only KK+. It's harder for me to assume that his 3bet ai range is only KK+ than it is to assume that his range is QQ+.

Against a known player that is anywhere close to tight/passive, it's an easy fold.

Others know way more than me though, so let them chime in.