Did you test a specific 2-cell version against a different specific 3-cell version? Or did you test a specific 3-cell light with 2-cells against the same light with 3-cells? If yes to the latter then it proves only that the 3-cell has great regulation. It doesn’t prove whether the 3-cell runs 0.5A higher and therefore brighter than a 2-cell only model.
Actually have mentioned this quite a number of times before....here goes....
I've got the 3-cell version.
You can check thaicpf, they have got the 2-cell version and the current draw is the same as my 3-cell version running 2-cells. Lots and lots of info there. Their beamshots also indicates that a stock SR3800 is slightly brighter than the TF-3T6. The current draw of both lights in 2-cells config supports this. (the SR3800 draws slightly more current, assuming similar batteries). From my own data, this is also the case, both for throw data as well as reflected ceiling lux data. The SR3800 is not dimmer than the 3-cell TF-3T6, in fact it is 19% brighter.
I do not take any data from the manufacturers or sellers. Not even the Fenix TK70/official website.
For this light, 2 cell gives you about 2.1A range. 3 cell gives me about 1.4A. As you know a lot depends on the cells voltage sag. I have Panasonic NCR18650A, NCR18650, Solarforce 2400, XTAR 2600, TF Flames, Palight, Ultrafires, CGI etc... the first 4 do not sag much and gives me the lowest current. Since then i have gotten a new DMM the UT-58E and have been measuring with 4-inch long leads, but i haven't gotten down to measure all the cells in various configs.... coz this light is already utilitarian to me as i use it in shorty 2 x 18500 mode with the BIO IMRs. (discharged to be about 1050mAh). If you are interested in these cells, this seems to be correct as i get 40 mins run on the STL-V6 and 35 mins with the 18350. This light can run on 18350s as well if you want to pack the biggest amount of energy in shorty mode.
All in all i have purchased 7pcs of various triple XM-Ls now. I think I have over-measured all of them...time to enjoy now. LOL!
All sounds good enough to me. I enjoy it in either amount of cells but it’s certainly easier to carry with just two. 
So with just the main body and the tail cap (shorty mode) two 18500 IMR cells fit perfectly? This light runs well with just one 18650 and a battery extension but just not as bright (still pretty bright) as with 2 cells and I love it in shorty mode! ;)
Yep, both the 2 and 3-cell models can operate in shorty mode with just the "main body". It's the same thing with STL-V6. TR-1200 too i'd believe as the tube lengths concept is the same (but the tubes do not fit, these triple XM-Ls front part of the tube has walls that are nearly 5mm thick!). 2 x 18500 cells fit perfectly and the tailcap screws on 100%. With 3 x 18350 IMRs it just leaves a wee bit of 2mm gap as it can't screw on 100%. 3 x 16340 might work if you don't mind taxing the cells as it's probably runs till 1.6A as the cells discharges. A bit hard for the TF Flames which i have measured to be 781mAh at 0.5A and old4570 measuerd 795mAh, so that's 2C. But still doable for 3 min runs.
And can somebody please confirm that the 3x18650 version in shorty mode works well with two regular 18650's? And would two XTAR 18700 cells fit?
Thanks!
Shorty mode won’t work with two 18650 cells. You will need the shorter of the two extensions. One 18650 will work in shorty mode with a 3/4"-1" battery extension.
Two XTAR 18700 cells are snug but fit well...again, you will use the shorter of the two extensions.
Thanks very much! So in this picture...
... for 2x18700 I would use the head + the body at 5-o-clock + the extension at 10-o-clock?
And for an extender to use with 1x18700 I could use a stack of dimes or pennies or something?
Thanks!
Correct on both accounts. 
Thanks Johhny.
You are always welcome, my friend.
I bet you could mount them in a panel and give those searchlights a run for their money !!!
TK70 in the middle, 3 DRYs around the edge, and a few thinner TFs and SRs to fill in the spaces....BAM !
Yes the TR-3T6 3 x 18650 model will run either 2 x 18650 or 3 x 18650’s not sure about 18700 as I don’t have any but the extra extention tube to run 3 x 18650’s is made from two tubes about 50 mm or 2 inches long (once screw down) so you could remove one of them if needed to make it a tad shorter and you can move them around to make it sorter or longer depending on how you put it to gether plus the tail cap has a brass plunger that can take up some of the slack or pressure if that makes sense
Very good, thanks benckie for the reply!
confirm can for both questions. Spring loaded inside.
Good, thanks 2100!
Ok to make it even clearer, the 3-cell version of TF-3T6 has 1 main tube + 2 extensions. 1 extension is shorter, and 1 extension is longer.
Those 2-cell version from Sky Ray 3800, 818, 850 and TF-3T6 have 1 main tube + 1 short extension.
All lights with main tube only can take 100mm cells. Eg 2 x 18500, 3 x 18350 (but with 2mm gap). Make sure your light can take 3 cells voltage first. So far I know TF-3T6 and the less popular SBOMZ can do that.
All lights main tube + 1 short extension can take 2 x 18650.
All lights main tube + 1 short extension + 1 long extension can take 3 x 18650.
I get 50 mins on 2 x trustfire flames and 80 mins on 3 x trustfire flames that’s on high with constant running with no dimming. on off running should give more as the rested cells rise back up on voltage.
Guess we can do a guesstimation. 2-cells 2.2A avg you get 70 mins with 2600 cells. Definitely less if you want full regulated output.
3-cells 1.5A avg you get 107 mins with 2600 cells. Definitely less if you want full regulated output.
Try not to push the cells too far, not for more than 15 mins after dimming. You may not be able to recover the cells.
SR3800 with higher draw (eg 2.8A avg) got me exactly 53-54 mins before LV cutoff.
I have not run in it dimming yet on my TR-3T6 3 x 18650,s that’s why i like being able to run 3 cells (above post i said domming dam iPhone) but on my TR-1200 I’ve run into dimming with cheap ultrafire greys that only took 900 Mah they have been scraped cause they where crap
Yep Benckie...3-cells, running in low mode of about 300 lumens and yet able to garner 20% more eff than single XM-L, very long times of a couple of hours, it's difficult to run the cells down unless in a real emergency.
Trust me the LEDs definitely won't die on you. Get 1 more additional driver and tailcap/switch (cheap) and you probably have a light for a good couple of years. Easily till the next Cree new pdts. :D