is a 3 cell maglite strong enough to 223?

If you mean an actual aluminum Maglite housing, not really.  At least not if you intend to fire more than a few rounds without letting it cool.

If you mean the size of it, but with a proper steel or titanium tube, then yes, but you really want more volume than a 1-1/32 ID unless you plan to make the thing really long.

*For some reason I read "3 cell" as 3 C-cell.  What size do you have in mind, Stoney?  I typically use 1.5" OD .035" or .070" wall gr. 9 Ti for 5.56 cans, depending on how I'm rating it, with a length between 5" and 8", 7" being kind of a sweet spot with that diameter for quiet enough to skip ear pro for outdoor use but still fairly compact.

The d cell battery itself measures 1.297. The id is 1.348. The wall thickness in the thread area is .070.
The od where not annealed measures 1.550.

The data indicates the tube is plenty strong for a 223 at a std range sight in rate of fire or for hunting but not for competion shooting.

Your better option for an aluminum tube and lower cost would get a 2024 tube threaded 20 tpi at each end so you could use std solvent trap.parts.