Hi, AH …
You say you already own PL3 - but you don’t say whether it’s the Elite version, with integrated View Point & FilmPack … but, I will assume so for now.
The Nik Collection includes a group of tools - some of which are duplicated within PL (such as Perspective adjustments, Noise reduction, Body+Lens-specific sharpening, B&W conversions, etc); with the benefit that you don’t need to export to the Nik tool to use these features and you thereby retain a complete and uncomplicated non-destructive workflow (by keeping everything within PL).
Given that, the one Nik tool that I don’t find is replicated within PL is Color Efex Pro (CEP) … That’s the only Nik tool that I regularly use (an occasional other is Silver Efex Pro - - all others I consider to be redundant) … For me, tho, it’s worthwhile owning the Nik Collection just for CEP alone.
Note: If you are using CEP then there’s a simple means of maintaining a non-destructive workflow between it and PL (albeit, with some minor manual steps) that does not involve using the new, but cumbersome, “loaded TIFF” method. See here for details.
Hope that helps …
Regards, John M