We usually only get the initial advance bookings in 3D and are allocated one "print" (In this case a hard drive as my current theatre is completely digital). We then take care of the advance show times for the midnight showing(s) and up until Thursday of the following week, for just that single print. Most films that are in 3D are marketed as in 3D so the majority of advertising is spent on hammering that point. Thus if we're allocated one print to schedule, it's a 3D one.
So my local theater in ct is only doing one midnight screen, in their smaller auditorium, in 3d. No 2d until next day, and no IMAX until next day. Wtf?
It isn't until closer to release that we receive more booking information (I.E. how many prints we will receive, if we will be receiving digital bookings in addition to the 3D ones, etc.) thus 2D (35MM or Digital) show times usually aren't added until a week prior, when we post the bookings.
Very rarely have we not had 2D shows to complement our 3D shows. I believe the only time we haven't in the past year has been for the 3D re releases (Lion King, Beauty & The Beast, Titanic, etc.)