Spec-ops Brand has a decent holster that works on a belt, vertical or horizontal( a good way to carry a light as large as the M3) on a pack strap or LBE with the MOLLE or RACK system etc. and it features what they call the "Shine-Thru" feature which is a nightvision friendly blue lens in the bottom that allows you to shine a less bright, concentrated blue beam out the bottom of the holster for map reading etc. This is the Spec-ops brand not the Special Operations Equipment brand.
Blackhawk also makes a horizontal pouch that has loops for extra batteries that fits the M3, It's called the Duty Surefire Accessory Pouch and looks a heck of alot better than their Surefire Millenium M3 holster( I think the Spec-ops Brand holster is the better of the two nylon holster type) The Surefire Accessory pouch has a full closure and probably the best protection. I don't like the single vertical belt attachment option of the SOE gear stuff as I like to put my light on my pack straps sometimes I also don't like the big *** side release buckles on the SOE.
I carry my C2 in a Sidearmor hard carrier and wish they would release a model for the M3. These are pretty expensive at around 30 bucks a pop, but I like the tension adjustment and bezel down carry. I make custom kydex and concealex sheaths for knives and one for the Inova X5T and haven't worked the bugs out of carriers for Surefire lights, but will soon hopefully.
Later,
TSG