If the router is acting as it should, and all your machines are sitting behind it (the router is acting as a firewall) then it is (relatively) safe to enable netBIOS / MS file sharing on all three.
What you do, is under the properties of NETWORK NEIGHBORHOOD on each Win98SE PC, "ADD" the "Client for Microsoft Networks" service.
Once each 98SE machine has that added, make each machine part of the same workgroup (found in the identification tab of network neighborhood properties)
Once that is all finished, you can share directories you wish other PCs to access. One neat thing about netbios + tcp/ip under windows, is that you can use netbios computer names to initiate a tcp/ip connection in games. So rather then putting in, say, 10.0.0.3 to connect to your other machine, you could put in "UGLYBOX" and it would resolve it properly.
If you want to have blind full access on all machines, share the root of C: on each machine and all other partitions you want to share.
Note: IF you do this, ensure that your router is filtering ports 135-139. Any machine that has netbios bound to tcp/ip in 9x, and any machine that has netbios bound to tcp/ip on an external adapter in an NT os, that has those ports visible to the internet, is extremely insecure and can potentially have its shares visible to the internet.
If you have trouble with this let us know and I'll give you a detailed walkthrough.