So the vet said 3, but it's quite obvious you are carrying more; so they wait till you've had 6 then take you out for a wee, clean your bed and then no 7 arrives!
A little girl late for the party, but she seems Ok. 4 dogs and 3 bitches, and you never really know it's over till it's over - do you!
Seriously, I wouldn't go to a vet for a pregnancy scan. No disrespect, but there are people trained to do a better job, a specialist in the field can give you a far better idea what to expect.