>The pup had to be minimum of 3 months old to have her first vacc,
Yes, that's correct; rabies vaccines are given from 3 months of age. For the pet passport a blood sample is taken 3 weeks later to test for immunity; if the levels are satisfactory then the passport is issued, the dog can leave the country but can't return till 6 months
from the date of the blood test.
If the level of immunity isn't high enough, however, the rabies vaccine will be repeated and another blood test taken three weeks after that for testing, and I've never known a second one to fail. This does mean that all this takes time, and the pup will be at least another month older before it can return.
>Are puppies normally required to only have 1 Vaccine for rabies?
In cases where it's absolutely vital that the pup passes a second rabies vaccine is sometimes given two weeks after the first, and a single blood sample taken two or three weeks after that. It's unusual for that to be done though.