Network Problem

I have a Mac Pro 1,1 (Mac OS X Version 10.6.8 Snow Leopard) and a Mac Mini 3,1 (Mac OS X Version 10.6.8 Snow Leopard) and a iMac 3,1(MacOS High Sierra Version 10.13.6) In the Finder Window under Shared I only see my girlfriend’s iMac 3,1(MacOS High Sierra Version 10.13.6) and that’s all. I don’t see the Mac Pro or the Mac Mini that are running on (Mac OS X Version 10.6.8 Snow Leopard). Why can’t I see these 2 machines that are all on the same network. Shouldn’t they show up in the iMac Finder Window Under Shared? Well, they don’t!

Also, on the Mac Pro and Mac Mini in the Finder Window Under Shared I only see the Macs running Snow Leopard. No sign of the iMac running High Sierra. Is there something wrong with my network or are the two Operating Systems incompatible with one another? What’s the deal here? In my work I need to get on the network and be able to grab files from either of these machines when needed. Somebody please help me here. I don’t know what the **** is going on with these 3 machines