You can credit United flights to your US account until 3/30/2014, just as you have been.

You will be able to credit American flights to your US account starting in "early January" -- that date is rumored to be 1/7, but that hasn't been officially confirmed.

So in short -- from now until "early January," you should fly US or United. From then until 3/30, you can fly any of the three; after 3/30 you should fly US or AA.