I also have licenses for FP5 Elite and Nik3. After testing the trials of FP6 and Nik5 I don’t think I need those latest versions. That’s the reason for asking if the latest PL5 Elite is compatible with those older add ons.
PhotoLab 6 will be released in a bit more than a month. Probably during the third week of October. I would wait for that release before purchasing. If you purchase PhotoLab 5 now you might have to pay an additional upgrade charge if you want to upgrade to PhotoLab 6. I believe in order to get a free upgrade to PhotoLab 6 from PhotoLab 5 you have to have owned version 5 for no more than a month before the new version is released.
The Nik collection 3 and FilmPack 5 should continue to work just fine with PhotoLab 6. There are several useful new features in FilmPack 6 Elite, but if they are not useful for you there is no reason to upgrade.
I don’t have Nik3 but I do have Nik1 however I’ve never tried to use Nik1 within PL5, I only ever use Nik1 (and then only really SilverEfex) as a plugin to Affinity Photo.
I used to have PhotoLab 5 and FilmPack 5 together (on a sale I did upgrade to FilmPack 6, which was not necessary, I was feeling generous). Nik v3 runs just fine from PhotoLab 5. From what I can see, more reliably than Nik v4 or Nik v5. On an M1 Mac, Nik v3 just stays in its lane as a Rosetta app and works. Sadly, it looks like DxO removed a bunch of compatibility in Nik v4/v5.