The reason you see "3 Yr USDA Licensed Vaccine" marked on rabies vaccine certificates is because the vaccines you are using are products licensed by the USDA to be valid for up to three years (for example the Defensor 3 by Zoetis). This is different than the next due date for a vaccination and these dates are separate fields on the vaccination certificate.
The 'Next Vaccination due by' field (or the date you listed the next scheduled vaccine in Shelterluv) is highlighted in yellow below.
And the length of time that the product you used is licensed by the USDA for is highlighted in red.
Take a look at this article for more information on how rabies vaccinations work: Rabies vaccine certificates.
If you do actually use a product that is only licensed for one year by the USDA, just be sure to select that product and your vaccination certificate will reflect "1 Yr USDA Licensed Vaccine."