I'll be 100% focused on BF3 (I'm currently playing MoH).
My suggestion is to perform a smooth migration, as opposed to a sudden cutoff, to avoid losing players. Start with 30% on BF3 and gradually increase it to 100% over time (say, 10% more budget on BF3 every month, and cut back on BC2 and MoH).
The thing here is that BF3 will probably require a pretty decent graphics card. Also, the game itself won't be cheap during launch and some people might wait for steam sales and such.
With that said, there's nothing better than monitoring the numbers and make decisions based on empirical evidence.