SBC DSL Has always used a PPPoE Client to connect with Username/Password. Unless you have one of the new SpeedStream modems that has I believe 5 lights, You will have to have a PPPoP Client to connect. SBC for sure and some other DSL providers are not like Cable where it's always there with DHCP. The new Speedstream modems they have been using here in Calif at least, have a built in PPPoE Dialer and you just connect with your browser usually to 192.168.0.1 or 1.1, forget which they use as I have the older modem, and put in your username and password. Check the manual that came with your SBC DSL.
In OS 8.6 you as was said above, go to the TCP/IP Control Panel under Control Panels and set it to Ethernet and DHCP. I do not think OS 8.6 has a PPPoE Dialer built in. OSX does.
Exactly what G3 do you have? Gray to means the G4's. The Tower G3s were either B&W or Beige. You said Gray so I am confused as to what system you have. If you have a fast enough G3 I would suggest trying OSX. It will work a LOT better on the Internet!
I hope nothing above confuses you. I have been using Macs on SBC DSL for years along with PC's but I use a router and it does the PPPoE connect for me.