HOney- that shot they all need to get it at the same time. The reason- even with one shot there is some resistance, but if the other 3 that didnt get the shot, do get distemper, the ones vaccinated may get it too. Do all 3 with the shots- it should not be over 7 -10 dollars a pup. Call your vet for advice. There is one other suggestion I can make, but would rather you call the vet.