Although you could definitely put linux, OS X Server (10.x preferrable, for both samba/SMB and applefile/AFP) or OS + ASIP on the b/w G3, I'd say to consider doing an apt-get install netatalk on the Ubuntu machine, and of course setting up Samba sharing. This will let both your MBP, your Windows PC and your older appletalk-over-ethernet Macs talk to the linux computer and you can store files there. It's what I did for awhile with a small old PIII laptop running Debian 5 Lenny.

The biggest concern is how much storage you can stuff inside it, and the b/w G3 could take a SATA card which would mean "any modern disks."