Define "won't take it" (Win10).
It depends on their performance level vs the tasks you might find them useful for, whether they are worth bothering with.
In many cases, a Winxp box is just too slow for modern uses, but a 2008 build, is not at all a WinXP era build. Systems of that age could have dual or quad core and 4GB to 8GB memory (if built well for the era) and fully capable of most common things people use computers for, just not very demanding apps.
How about Win7? Basically if you have any need that you could put a system to task doing, you then see what the newest OS is that it will be able to use to perform that function, if WinXP won't.
Contrary to what someone else suggested, it's not at all much of a worry to put a WinXP system on the internet. You just have to be aware of points of entry, for example, that it is behind a router, and that your browser is hardened and scripting disabled. Don't use it for data sensitive things like banking, and have an OS partition backup so you can just nuke it from orbit if you ever have a problem, which is pretty unlikely unless you start installing warez from untrusted sources. Otherwise it is mostly about making sure that your browser limits exploits, which means it won't work properly on many modern sites so-configured.
I'm thinking more of WinXP era here, if you run Win7 which a 2008 box is likely to have drivers for, you can run a modern browser version. A Win10 box is MUCH less secure than a Win7 box in the hands of an aware owner, because the primary threat become MS updates themselves, preventing it to even boot or loss of functionality.
Linux, meh that's silly unless this is a single purpose system, set up to be a firewall or caching web server or fileserver, etc. For general purpose use like a normal desktop PC, there are far fewer reasons to choose linux than not to.
Ultimately the devil is in the details. You probably don't need all 3 systems, but might want to keep one around as a backup or mix and match components to end up with the best of all 3 combined into one system. For example turn one into a NAS, but keep it functional enough you could use it for your everyday activities if you needed to because your reliance on Win10 puts you at risk of a sudden loss of those systems at any moment.
As far as what to recycle it depends on whether you build your own systems. Having a spare case and viable PSU lying around is very handy when it comes time to upgrade, you can build into that case while keeping your main use system up and running, used for everyday things while you test, install software, configure, etc, so there is a seamless transition from current daily use system to the replacement system.
Ultimately if you don't know what to do with them, then you don't have a use for them and should not recycle but rather see if any local charities, goodwill, or needy college students want them, but of course it would be worthwhile to see if they can run Win7 at least, but if they are free, then the recipient can do that work.