L3 is a microkernel running on Intel's x86 computers. L3 is designed to be "very lean and features fast, message-based, synchronous IPC, simple-to-use external paging mechanisms and a security mechanism based on secure domains (tasks, clans and chiefs)." It is the predecessor of L4, another microkernel with similar design goals.
L3 is also the name of a full operating system based on the L3 microkernel.
Both were developed by Jochen Liedtke and his team at GMD's SET institute in 1988, as a successor to their earlier Eumel operating system (also known as L2).