What is an F 3 mean?

Referring to "an F3" can mean a couple different things.

It may refer to one of a number of plane models including the Panavia Tornado, a fighter jet used by the RAF.

It may also refer to a level on the Fujita scale, a system of rating the severity of tornadoes. On this scale the weakest tornadoes are rated F0 while the strongest are rated F5. A rating of F3 indicates a strong tornado capable of leaving well-built houses mostly destroyed.