IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. This is generally a local area network technology with some wide area network applications. Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable.
802.3 is a technology that supports the IEEE 802.1 network architecture.
802.3 also defines LAN access method using CSMA/CD
Read more about IEEE 802.3: Communication Standards