It is always in play, but you will only be able to take advantage of it if there is a shortage of suitably qualified 3 year F/O's.
For instance if you were on the airbus at present, where there is a backlog of 3 year guys waiting for a command, then you would not get offered an accelerated command.
On the boeing at present you probably would.
I am not sure of the rules applying to changing fleets for an accelerated command. Someone else may care to comment.
V