Diff service isn’t included as part of the service schedule and there’s always been some difference of opinion as to how often it should be done.
Every 3 years is, i think, the manufacturer’s recommendation- i think that’s probably unnecessary- unless you drive the car hard.
I’ve not heard reports of diff failure.
It isn’t expensive- I paid something like £80 when I got mine done-
I doubt they would’ve done any more than drain and refill.