This is a silly little game that vets and the manufacturer of the vaccine plays.
In the case of Defensor I and Defensor III (rabies vaccine by Pfizer), it is the exact same vaccine labeled differently to meet the needs of dogs in different states with different requirements.
I'm not sure if all companies do the same.
If you live in a 3-year state, it does not need to be given yearly.
Pup shot, then one year later, then every 3 years.
I'm sure the vet will insist that you get the 3-year vaccine if you only want to give it every 3 years allow the 1 year rabies would last for just as long.
It is a potent vaccine though, so whether you give the 1 year of the 3 year, there is quite a bit of potential for reaction.