The need for refresher training and appropriate frequencies depend on so many things.
How good the training
How complex is the site
What is expected of the fire wardens
How complex is the emergency plan
How often do they get to put it all into practice
What other level of fire related responsibilities do they have
Annual refreshers will be needed if for the rest of the 12 months the fire wardens never get to think aboout fire safety again. Or if their role as a fire warden is totally divorced from their day job.
But if they are regularly hands on, for example carrying out regular drills, hazard spotting, testing fire alarms, sprinklers, emergency lighting, inducting new members of staff, evac chair training etc then 3 yearly updates may be fine.