Only the installation of content/expansion packs should be time consuming. While it's pretty easy to just copy the one file over, you can also just reinstall BFD2 to get it back. It only takes a minute, tops. Just download it from your support account, and don't reinstall anything related to content. You can just move the file over manually, or I suppose the upgrade process copies it over. It's been a long time since I've upgraded.
Platinum Samples (any content) can be installed at any time. You'll rescan your content drives once you've installed BFD3 or installed new content anyway, so order doesn't really matter. I installed PS at least 7 years ago on Windows, and have since just copied it over to Mac, where it's gone through a few versions of both BFD and MacOS.
One thing you'll want to do once Platinum Samples is installed is run it through BFDLAC. It substantially cuts down on disk use and loads faster than the old BFD versions. It's not backwards compatible with BFD2, so if you have some reason to use that again, don't do it.