The C3 is a single medication that contains all three core vaccines. If your vet decides to give your dog or puppy any of the non-core vaccines, they'll do it at the same time as the C3. At that point the treatment becomes known as a C4, C5, C6 or C7.
Here's how it breaks down:
C3: A single medication containing all three core vaccines.
C4: The C3 plus a second medication containing the PI vaccine.
C5: The C3 plus a second medication that combines both the PI and Bb vaccines (the two "kennel cough" illnesses).
C6: Everything you get with the C5 plus coronavirus jab.
C7: Everything you get with the C6 plus protection against leptospirosis.