Two episodes, not three. Sorry.

Series 8 is 12 episodes instead of the usual 13. Possibly because A) the running time for a 13th episode is split between the extra time for Deep Breath and the extra time for the 12th episode, and/or B) the Christmas special is only a month and half after the finale, so they didn't have much of a break between filming the series and filming the special.

Of course, episode 12 could end on a cliffhanger that's resolved in the Christmas special, but I don't think that's very likely myself.