Three 123s are the length of two AAs. You would need to enlarge the ID of the Minimag resulting in an ID that would be dangerously close to the OD. For an alternate light, do a search on Coleman AA.
Even if there was enough OD to bore out to fit the 123's, it would destroy the threads where the tailcap screws on.
But don't feel bad, you're not the first one to think of doing this..
I did a write-up in this thread that describes how to build a 2AA/3-123 Coleman mod. The Coleman was chosen because it was the only economical light I could find that possessed the required attributes to make the mod work.