Ya I agree, there's no difference between a "3 yr." and a "1 yr" as far as what's in the vaccine. Lol, you're vet's just probably trying to make up for that income loss for the owners that choose every 3 years, the vaccine isn't costing him any more to buy unless maybe he's using a different "brand" for the ones labeled 3 yr.
It's the same price at my vet - they just do a quick education with the owners and help them decide which way to go based on the dog's exposure level
(they're a bit new to me and that's just what I was told when I said I insist on only doing every 3 years
).