I assume he needs a 17mm diam driver to fit Mattaus's copper heat sink. The Maxflex is too big at almost 23mm diam.
Serial XP-G at say 1.5A drive would give a total Vf of perhaps 10.5V or about 17.5W total draw. In theory, you could use an AX2002-based buck driver to run the series triple. The reality, though, is that thermal management of the waste heat from the driver and the LED triple is going to be a huge challenge. Even if the triple is 30% efficient, 70% of the 17.5W is waste heat, or about 13W. And even if the driver is 90% efficient, it generates another 2W of waste heat. 2W could be manageable in a big 23mm board that is designed with thermal management in mind, e.g., thermal vias for key components like the switcher IC. But these cheap Chinese boards don't have anything like that. They just use surface mount components sitting on a highly thermal insulating MCPCB. The best you can do is probably to try to glue small heat sinks to the switcher IC and Schottky diode. The problem is further complicated by the fact that the P60 heat sink also has to deal with the waste heat from the LEDs. The driver is going to see some part of that massive load.
If you can get a triple XP-G2, then total Vf at 1.5A drive could be about 9.3V, or about 14W draw. If you cut the drive current back to 1A, power draw could be about 9W. A 90% driver would generate about 1W of waste heat and the LEDs would throw off about 6W more. IMO, it would still be a hot running light, but you probably could get away with it as long as you don't expect to operate the light for long, continuous runs.
Basically, since the AX2002-based drivers are cheap, you might have a go at a full power, 1.5A serial triple and if that croaks (hopefully, it'll be the cheap driver, not the more expensive triple LEDs), dial it back to 1A drive.
With Vfs of around 9V to 10V, you'll probably need to use 4xLi-ion to make sure you have enough Vbatt to stay in full regulation.
