Cat 3 is currently still in use in two-line telephone systems. It may be used for token ring networks, or 10BASE-T Ethernet. The seldom used 100BASE-T4 standard, which achieves speeds of 100 Mbit/s by using all 4 pairs of wires, allowed older Cat 3 based infrastructures to achieve a much higher bandwidth. Cat 3 is compatible with Power over Ethernet.
Note that unlike Cat 1, 2, 4, and 5 cables, Cat 3 is still recognized by TIA/EIA-568-B, its defining standard.
Internal Telephone Cables is for use in networks within telephone exchanges, commercial switchboards and interconnecting wiring systems. 25 Pairs, 24 AWG (0.5mm), Copper wire, PVC insulated, twisted pairs, unshielded, PVC outer jacket