Offer them for free locally, either whole or part them out.
What I do is try to trade my parts for ones I need, in the end it saves stuff from the landfill and gives things new life.

Nothing there really lights my fire. That being said, machine #2 wouldn't be bad as a retro-gamer. #3 i'd pull the processor, ram and scrap the rest. #1 i'd probably end up pulling the ram/cpu and hdd and MAYBE the motherboard. It would depend on the boards features.

Value in $ is probably not much really, i got similiar whole pcs for 5-10$ at the flea market recently. However a retro pc enthusiast could certainly find use for those. One thing that quickly got my attention is that 3Com ethernet card in machine #2, depending on the model, especially if an isa, could be one of the few select cards supported natively by early windows ( w3.11, w95, w98.. ). Considering how hard and frustrating it can sometimes be to get a nic working on those systems, this one is worth verifying. Also, k-6 proc value seems to be on the rise, they will cost you more than a p4 if you try to buy them on ebay.

As others said before, its worth it to protect and value these things, not only because its fun, So many people are trashing old computers (sad) that they eventually end up getting rare and gain value.

The Super Socket 7 board caught my attention, till I noticed I'm pretty sure I have that same Soyo SY-5EMM, and I hate it with a passion. Very very finicky, but does work well enough with a K6-2 350, coincidence? 😜

Really, IMHO the only thing that might be of interests/worth are the cases. Any pictures?