Can you fairly have one rule for one and one for another?

[TheFantasticMrsFox]

DDogs 1&2 are working dogs who, until the arrival of DDog3 were never allowed on the furniture.
Ddog3 has clearly been allowed to sleep on the sofa and TBH that doesn't concern me too much, she's small, short haired and rarely gets muddy.
So is it fair to allow her up but not the others? I know that dogs have no concept of fairness or injustice but DH says that we cannot have one rule for her but not the others.

Hate to break it to you, but there is some research that would suggest dogs do have a sense of fairness and can feel jealousy. Here. Although, this article would suggest that although they do feel envy their concept of it is far less complex than it is in humans.

Having said all that, we have an 'only by invitation' rule regarding dogs on the furniture. Mine only get to come up on the sofa very rarely and then usually only one at a time and seem fine with this. They also understand that at our holiday home (which is owned by a dog owning family member and has throws on all the sofas) they are allowed up on any of the three sofas whenever they like, yet they never try to at home - so clearly understand the distinction.

My father allows one of his dogs a whippet up on the sofa and my dog a jackhuahua but does not allow his other dog a Japanese akita up as she is too big the whippet also sleeps on his bed but the akita isn't allowed and sleeps in her own bed in the bedroom.
The akita came first so is used to this, so it do'snt seem to bother her she knows she's not allowed and never tries even when the other dogs are up there.
I think as long as the other dogs don't seem bothered or jealous about the new dog being up there it should be alright.