Like the thread title says, will the driver in a flashlight that takes 3 x AA automatically accept a 26650 since the voltage is similar? It would have to be in series, correct?
I think it should yes, the cyclone c88, is sold accepting both, and looks like the kaido drivers:
http://www.intl-outdoor.com/amc71358-5mode-circuit-board-nanjg-105-p-216.html
Nanjg 105C driver you mean, KD drivers are a bit different (different ways to set modes etc). Can't say about "any" thought - some of 3xAAA (and most likely 3xAA) cheapo lights are direct-drive (by relying on user using alkalines that can't give out a lot of current) therefore it could get fried by capable-of-massive-current 26650 cell.
I would be careful if it's a direct drive light running on 3AAs that top out at 2.5 amps and you stick a 26650 that delivers 10 amps in there it could go poof. Obviously the ones with the Nanjg driver wont.
I mean flashlights that are regulated, not direct drive.
So why would a direct driven flashlight not be capable of operating on 26650 if they can operate on 3 x AA? Isn't it just whatever the LED can handle? What about all the flashlights that already operate on 18650/26650 DD? What makes them different? The emitter remains the same. I'm still a noob 
About direct drive - if you DD a 1.5A capable LED with 3xAA alkalines, which really won't supply much more than 1.5A, it'll be fine. But if you use 26650, which is capable of tens of amps, it'll simply burn the flux out.
Cool, thanks for the education. I would want it to be regulated, anyway. I'd like two or three modes. I was discussing this concept with a friend today and we'll probably try to build it.