PL-3 or POS-PHY Level 3 is a network protocol. It is the name of the interface that the Optical Internetworking Forum's SPI-3 Interoperability Agreement is based on. It was proposed by PMC-Sierra to the Optical Internetworking Forum and adopted in June 2000. The name means Packet Over SONET Physical layer level 3. PL-3 was developed by PMC-Sierra in conjunction with the SATURN Development Group.
The name is an acronym of an acronym of an acronym as the P in PL stands for "POS-PHY" and the S in POS-PHY stands for "SONET" (Synchronous Optical Network). The L in PL stands for "Layer".