Legal move?

[Juulie Downs]

Player A3 is standing in the free throw half-circle waiting to receive a pass from the wing. Player B3 comes up and legally establishes a guarding position facing A3 and right in front of him. B3 has hands up, feet well planted and spread, but not too widely. A3 is shorter (by quite a bit) and quickly steps to A3's left and then back in behind B3, receives the pass, and dribbles down the key for a quick shot. B3 was startled, and didn't react. I think he thought that A3 was completely clueless, and didn't anticipate any real action. Now the question: If B3 was fast enough, could he have tried to step in front of A3 and take a charge? A3 stepped forward and to the left, but since they were both completely stationery prior to A3's move, does B3 need to leave any time or distance? what legal defense did B3 have?