If you are on the blinds and the BU/CO raises his position 100% of unopened pots, it's probable that he's opening light very light and you should open your 3-betting range to a lot of suited connectors, broadways, ace highs, and even some junk from time to time. It's awful to play pocket pairs out of position given most of the time you won't improve, so I think it's a great option to 3-bet pocket pairs (55-99) from the blinds unless if the raises come from UTG or UTG+1. Your opponent should be folding most of his range if his opening ATC late position.
Also, to me (tournament/cash game).
3-betting with position post flop: 2.25x/2.75x original raise.
3-betting without position post flop: 3.25x/4x original raise.