Sun equipped all Sun3 machines with ethernet interfaces. In these days the thick yellow cable (10base5) was state of the art which could stretch along 500 meters (with repeaters 1500 meters). Every Sun3 does have the 15 pin AUI ethernet port. An up to 50 meter long transceiver cable connects to the ethernet transceiver which sits directly on the yellow cable (vampire).
The modells 3/50, 3/60 and 3/E have an additional BNC connector for the black thinwire (10base2) which can only stretch over 186 meters but is much cheaper.
Today, no one uses the yellow cable anymore. Sometimes you can still find the black thinwire, more often the newer twisted pair ethernet (10baseT). But there are cheap adapters available to connect 10base2 or 10baseT to the AUI port.
An AUI-BNC adapter can be built from old vampire transceivers as the second picture shows.