Unless there is a fault, you could supply an appliance with a 1000 amp fuse even if it only needs one amp and it will work OK. The fuse/MCB/RCBO/RCD are only required when there is a fault, the idea is, when there is a fault, the easy to replace or reset protective device removes the supply automatic without causing damage to the system.
So to work out protective device size, we need a risk assessment as to what can go wrong causing over current. So if a stalled motor will burn out, we set the protective device as a current less than the stall current, but higher than run current.
So on central heating although the motorised valve stalls, it is designed to stall, so does not need protection from that, fans and pumps in the boiler use a very low current, so to protect them a fuse much lower than 3 amp would be required. So in real terms I personally can't see why a 3A fuse would be fitted? Only time I have seen the fuse rupture is because some one has fiddled with wiring in some way, be it drill through a cable or make wrong connections, and it can be said we don't need to protect against some one doing something silly, it is up to people not to do anything silly.
However where ever we live, we are brought up to expect a certain level of protection against doing silly things, and if we don't follow the convention of that country then we get danger, so if for example all safety switches are red or yellow and we find a black switch we do not consider removing that switch will cause danger, but we would not dream of removing a red or yellow switch. So if some one use to UK working practice works on a German machine where the safety switch is black, then as happened to me, one can be involved in an accident that means you never work again.
So since in this country we should fit type A RCD and a 3A fuse in a FCU that is what we should do, and it does not matter one little bit what the daft Germans do.