[kerr]

This is pretty simple guys. Ask yourself if the total amount a player has to call constitutes a raise of his bet? If so, then he can re-raise.

A bet 600. He now has to call 3600. 3600 > 600 thus A can raise.

If A flat calls, B has a further 1200 to call. 1200 < 3000 thus B cannot raise.

However, if B was a forward-thinking tricky fellow, and could predict what his opponents would do, he would have first raised to 2400 rather than 3000, allowing himself to re-raise and isolate A when the betting got back round to him.