Torchboy-so far I've been running 9AA in series to 3xCrees in series. The tailcap MM amp readings have been around 1.1 with rested Eneloops.
That implies the Vf of the LEDs is quite high, so it sounds like using just one 3xAMC7135 board to drive all three LEDs in series might be appropriate. That arrangement has the advantage of having just one regulation voltage drop for the three LEDs instead of one voltage drop per LED. (Theoretically an extra 0.24 V.) The AMC7135 board drives just one LED, as per usual, but the other two LEDs are put in series with that setup to drop the extra volts, the same as you might do with a power/signal diode or two to drop multiples of 0.6-0.65 V. As the set current of the board is only a few microamps all three LEDs get very nearly identical current.
One of the problems with doing this is the variability of the battery voltage. Each NiMH cell has a working voltage range as it flattens which will be three times as much for nine cells in series (driving three LEDs) as for three cells (driving a single LED). However, with the right setup of LED Vf and maybe signal diodes it can provide good protection against over-voltage with fresh cells and come very close to direct drive as they flatten.
Perhaps you could crunch some numbers on it. Do your friends like to use primary batteries instead of Eneloops? What is the combined Vf of the three LEDs in series?