MB are not required for FP+, but MB are also assigned to an account and not any kind of specific ticket, so even then if you don't scan it for entry, you should still be able to scan it for FP+.... if you don't want to scan your magic band for entry (though as long as you don't enter before 4, are on the correct day for the ticket and enter where they tell you to for the party, it will scan it as your MNSSHP ticket and not any other day ticket) tickets also hold the RFID chip that uses FP+.
And in regards to 3 passes, if you do only 4 - 6, you may not be able to book 3 passes as the last time will be about 5:30 so it just doesn't allow the timing for 3 (4-5 & 5-6 [or 5:10-6 etc]) but you *may* be able to book something like 3:30 - 4:30; 4:30-5:30; 5:30-6 but some have mentioned that last year they had their first one cancelled because it was before the "valid" time. I don't recall this happening for my family at the Christmas party so YMMV