Two parallel CR123A batteries DC voltage converter
I wouldn't worry about a battery exceeding 3.3 V.  The voltage comes from the chemistry.  Someone would have to use significantly different chemicals to make a higher voltage.

You might consider putting the batteries in series, then using a buck converter.  By the time the cells get down to 1.65 V each, there is so little energy left that you're not really getting any advantage draining them to 1.2 V.