The way I look at it:
ACT I = Episodes 1 and 2
ACT II = Episode 3
Then it kind of goes back, since the jump forward in time requires everything to be set up again:
ACT IB = Episode 4
ACT IIB = Episode 5
ACT III = Episode 6
So basically, the entire OT is Act III, but it has its own 3-act structure within.