Whichever service it is, if year three service means the car is three years old, then it’ll also be due a brake fluid change which is chargeable in addition to the cost of the service.
If the car’s been serviced annually and it’s now coming up to three years old, it’ll be a minor service (as @jv has said, they alternate between minor and major). However, if you’re on flexible / long life servicing the car could go up to two years / 18k miles before it indicates a service is due, and because of the longer time/distance since the last service, maybe each long life service is treated as a major one (I don’t know if this is the case as I’m on annual, time and distance servicing).
If it is a minor service that’s required and the online booking system is forcing you to book a major service, maybe it’d be better to speak to the dealer’s service team rather than use the online booking facility.