The instructions on
this page for Classic Mac Networking are extremely long and comprehensive, and it's worth reading. I also have a couple of Mac books from the mid-1990s, like Macintosh Secrets, which have networking chapters.
If you enable File Sharing on your iMac G3, your Mac SE/30 should have no trouble connecting to it. But you will need an Ethernet card in your SE/30, or you will need another Macintosh, such as a Macintosh LC (this is what I used) or a Macintosh Quadra, which has both LocalTalk and Ethernet ports. This Macintosh will run
LocalTalk bridge, and it connects your Mac SE/30 to the iMac G3, from LocalTalk to EtherTalk.
Regarding the original poster's configuration:
As the Mac mini is running OS X High Sierra, that's trickier, as that version of OS X does not have Apple Filing Protocol, so you can't easily use AppleShare between it and your Power Mac G3.
Netatalk will add AppleShare back to the Mac mini so you can host an AppleShare server on your Mac mini and mount it on the G3.
With the afore-mentioned Macintosh running LocalTalk bridge, you may be able to get the AppleShare volume on the Mac mini to appear in the Chooser on the Mac SE/30.