pallentxThree CR123s may be safe, as that should stack to 9V (Less under load), and it is meant to handle 2 RCR123s, which could be as high as nominally 7.4V (8.4V peak). Most circuits have safety factors built in to ~20% so the extra 1.5V shouldn't hurt much. The whole flashlight acts like a heatsink for the LED, so overheating shouldn't be a problem. I would avoid trying three RCR123s, though, as that would get up to 11.1V (12.6V peak), which is a bit toasty.
XenoMuseYeah, I don't think I'll risk it. Running with 2 CR123s, the light gets pretty hot in your hand thanks to the solid metal construction. I don't think I'd want to drive it any hotter.
XenoMuseSeems like if you can theoretically use it for 3 CR123 batteries then you'd be able to use a an Olight 18650 3200, 3500, or 1800 mah protected battery.