Not a vet, but I've dealt with multiple dog households for years and years. And yes, it's not uncommon for dogs to show outward reaction to the death of a dog that was a long time part of their pack. One of the symptoms can be refusing to eat, among a lot of other things.
My first suggestion is to go get them checked for any physical problems by a vet. Although it would be extremely coincidental for them to have a physical problem, it's always important to make sure they don't. Once you rule that out, then you can start working on the other stuff. And at least that way you know you've ruled that part out.
In general, dogs live more in the moment than people. Usually they'll adjust and move on fine. Try different foods or treats and see if they take to those. Even if you don't ever give them people food scraps, try that (obviously make sure it's food that is not going to harm them) and see if that helps. Give them lots of attention, familiar toys, and so on. Try to make sure they're getting good exercise each day. Things like that.
Edit - And one option is getting another dog, too. Assuming the'yre compatible, dogs usually react very well to the addition of another. You're in a good position in that you had three dogs, so the two left have each other still, but that doesn't mean adding another dog back in wouldn't help. It probably would. But obviously that's only if you want a third dog again - it's certainly not necessary, as your remaining dogs should be fine without a third.
