It isn't a '3 year apprenticeship'.
There is one way into the Armed Forces and that is to apply and go through the selection and training process. An apprenticeship is gained by successful completion of training and certain tasks during your first tour. Three years is the commitment you have to make to serve after completion of specialist training.
Yes, being in the RAF as a Firefighter will give you transferable skills to be a civil firefighter, but make sure you understand exactly what joining the RAF entails.