L3 is what F1 teams put on the pitboard during races. It means Lap 3.
Generally, drivers are told what lap they're on, or how long until their next pitstop, by radio. However a a pitboard (like the ones on the t-shirts) is also used to re-iterate the message.
An example might be like this:
L17
P3
Hamilton -3.4
Button +7.1
L17 is the lap number = Lap 17. P3 is position = third. Lewis Hamilton is ahead of you by 3.4 seconds, and Jenson Button is 7.1 seconds behind you.
Hope that clears it up
Edited: 15/03/2010 at 19:36