What does f3 mean on an oven?

If the make and model number of the oven had been given in it, this question would be much easier to answer. If you don't have the User's Operating Guide you could try two things: * go online to the oven manufacturer's web site, to see if they publish a Guide there, or

* call in at a local store which sells that make of oven and just ask them if they can tell you the answer - or they may even have a User's Operating Guide you could look at! You don't say which make or model it is so it's impossible to help you further here without knowing the make and model number of your oven.