IMO...
Any speedster capable of protecting the puck. This assuming speed overcomes coverage.
Offensive D likely would tear 3v3 apart due to higher options and flexibility of attack, speed and coverage. I assume at this point, it's about figuring out the best players to beat other players. However, at a later date, a coach or team might figure out a strong 3v3 system/strategy using combos of speed, coverage and attack rather than purely specific players capable of attack and speed.
On a side random note, worth contemplating a strategy for pulling out a goalie on the 3v3?