The first dose is always one year
Whatever product is used, a pet’s first rabies vaccination is treated as valid for one year. The booster given a year later can then be for one or three years depending on the product.
Products
Rabies vaccines are licensed as 1-year or 3-year products based on how long they were shown to protect. Many clinics use a 3-year product for boosters; some use 1-year products by policy or because a state or county requires annual vaccination.
State and county rules
Most states follow the label: a 3-year product means a 3-year certificate. A few states or counties require annual vaccination regardless of product, and the certificate will show a one-year due date. The rabies rules by state guide covers where to check.
Read the certificate, not the receipt
The "next due" date on the certificate is the one kennels, groomers and airlines use. If it says one year, it is one year, whatever the product’s name suggests.
Sources
Rules and guidelines change, and businesses set their own policies. Confirm with your vet and with whoever is asking.