KJo is definitely not an open fold from CO early in an MTT. How I react to a 3-bet depends on the sizing, the position of the 3-bettor and reads / HUD-data. I am more likely to call a SB 3-bet rather than a BTN or BB 3-bet, and of course I am also more likely to call, if I am getting a good price, or if the opponent is very loose and aggressive. If its a large 3-bet from an unknown, then I fold. A hand like KJo is somewhat problematic in a 3-bet pot because of reverse implied
odds. Meaning that when you connect with the board, often that just sets you up to pay off a better hand. Like it comes K83, and you end up losing a big pot to AA, 88, AK or KQ.