I’d put the 1gb ram laptops to server/kodi/retroarch/something mode and focus on the three decent machines for anything that requires a modern web browser, or add some ram. Porteus might be worth a shot if you’ve not tried it and want to push the Firefox on a potato idea.
I don’t think this is a one OS fits all situation, unless maybe Gentoo.
Why?
Triple booting is a pita, moreso if you don’t know how to partition a disk. I’d want any laptop encrypted, which adds further complexity to the triple boot.
If you wanna browse, research, watch videos and tinker just install a distro. If you wanna spend time switching your system off and on again over and over and over again to find out what’s working/broken go for the triple boot.
Docker could be worth a shot. You can ‘docker pull fedora/arch/debina/whatever’ and can play around with the base systems. Alpine takes up about 6mib so isn’t too resource intensive if you need to nuke it a few hundred times to get up and running.