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.
How bad is really bad?
AntiX is a good choice. Other option is a usb3 drive for each family member so everyone has their own portable AntiX on a stick.
MX is the related project with a more standard install and could be worth a look, the Fluxbox option should be quite light.
Each user could have a personal AntiX system on persistent usb3 and each system could have a bare metal MX Linux install. Just see what wins out via natural selection over time.
LXQT is another option for a full desktop environment that will run on a potato. If family members are mainly just users and you are admin, the base OS may not matter much. They could switch between a potato running Alpine and a good system running Fedora and if they are just logging into LXQT to launch browser, office, email etc the internal system plumbing is not gonna concern them.
Likely Mastodon.
Similar too Twitter/Bluesky and able to follow Lemmy, Kbin and lots of other areas of the Fediverse.
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.