F3 stands for frantic, frustrated, and female.

It's also the first hentai I showed to a bunch of 
my friends in college, and the exchange student 
ran in with a camera, took a picture of the 
monitor, and screamed "I Love America!"

F3 is very, VERY good, because it always 
surprises you.  It's not just 'oh, a girl in a 
sailor suit.  I bet there's gonna be 
tentacles...' which is around 50% of all hentai 
I've seen.