If you are going to run 3 XML, that battery holder isn't a good choice if you end up running the Maxflex.. It is for 4S1P which will make a 14.8v pack. For the Maxflex, you need the 2S2P holder.
Now, if you end up running the H6flex, the one you linked to will work good.
Make sure you pay attention to what type of driver you get before you decide on a pack.
You have buck, and boost type drivers.
With a buck type driver, the general rule of thumb is to make the pack out of as many serial cells as you have LEDs plus 1 cell.
So a 3 LED light would use a 4Serial pack.
With a boost type driver it is the opposite.
3 cells would need a 2Serial pack.
Read the tech docs for each type of driver and it will all make sense.
Also, that cable gland is REALLY expensive.
I almost ended up ordering the same thing and was pointed in the right direction by a member.
I ended up going all crazy and ordered 4 different types of glands, and several of each type.
I have a box full of just cable glands.
I'll send you one for the cost of postage. Just PM me.
Your link to the switch is not working for me.
Make sure the switch is momentary N/O, aka, normally open, momentary off (on), push it on-release it off type.
I've had really good luck with these switches I found on ebay............
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=290504434717&ssPageName=STRK:MEWNX:IT
of which I have about 50 of....hehehe
With that pack you would get about 1 hour 10 minutes running the H6flex at 3000mAh.
If you run it at 2000mAh you up it to about 1 hour 45 min, which is what I would do.
For optics, the XML is pretty floody compared to the XPG, so even the spot optics produce a fairly nice usable beam.
2 Laura RS and throw in another type of Laura, like a diffused or an oval.
Time to throw a monkey wrench in your plan...........
Do you really need 3 XML?
Thats alot of light.
2 XML is very bright as it is.
Troutie has some sweet 2 XML housing kits up right now. You may want to contact him.
http://www.singletrackworld.com/forum/topic/diy-900-lumen-and-1800-lumen-kits
If you do end up changing over to a 2 XML, you will need to rework your list completely.