Depends on the room, and the use (live, or recording). For live use, two dynamic mics close up to the vents of the horn compartment, at roughly 90°. I have the back panel of the horn compartment removed on my Leslies, as I like the visual feedback on Leslie speed, so I'll usually have the mics on the other side to minimize wind noise. Single mic close-up for the bass rotor.
In the studio, I have heard good things about using just two large-diaphragm condensors, on height with the horn but aimed downward slightly, halfway between the bass and horn. Try them about four or five feet away from the Leslie and spaced apart around the same distance. This only works if you have a sufficiently dead (or nice-sounding) room, since you obviously get a lot of ambience. On the last recording session I've done with the B-3 with 122 and 760 Leslies, we did a mix of four channels: the close-up, and a single condensor about four feet away, mixed to stereo.
sounded really nice, but we didn't really have time to experiment. Adding the slightly-distant condensor made a HUGE difference, making it sound a LOT smoother and more "natural".