I don't know how many of us use military symbols in our mapping. Of course Annual Volume 3 November 09 issue https://www.profantasy.com/annual/2009/november09.html has many and has the advantage of being integrated in CC3. It has a shortcoming (for me at least) of not being able to compose many of the symbols currently in use. It models "old" and "new" symbols defined in the US by MS2525 C/D and NATO APP 6 B/D. It provides the four types of units: Friendly/Blue, Enemy/Red, Unknown/Yellow, and Neutral/Green. Filled colors are limited to those, but for diagrams without a fill, there is a large selection. They can also be edited in some way in drawing programs.

The website is located here: https://spatialillusions.com/unitgenerator/

Attached is an example of some of my (incomplete) work using them.

I have not yet tried copying symbols from a separate document and pasting them into CC3. But they can be composed as *.png or *.svg which, by more skilled practitioners than I, can problem incorporate them into symbol catalogs.