I've only done a few tubs, but I always did #1. But that's mainly because I used Schluter stuff and that's what they recommend. It probably wouldn't work with a fluid applied membrane.
Though thinking about it, I'm not even sure how you'd do #2 or #3 in my installs. The flange sat flush against the studs and was about 1/4-3/8 thick. So if the backerboard overlapped the flange, it'd be bent out at the bottom unless I cut away at the back of the board or something. Unless you fur the studs out to be flush with the flange?
But basically for #1, the board bumped up against the top of the flange and it's thickness stuck out towards the center of the tub by about 1/8-1/4". Then when I put thinset on the wallboard for the membrane, I put a bead of caulking on the upper part of the flange to adhere the membrane to it. That creates a flat plane from board down to the lip of the tub, so you can just tile normally up from the tub with a small gap at the bottom which you caulk at the end.
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