Jeff,
I believe your father used artistic license on the front nose plate. In the painting, the locomotive is quite stripped of all goodies and the front plate would have been one of the first things to go. I feel he understood locomotives (the locomotive in the painting is perfectly proportioned) and felt the front needed an accent and he painted in a nose plate and arbitrarily put the Number 3 on it. The locomotive could have been on an abandoned siding at the mill. It also spent some time in a junk yard and it could also have possibly been painted there.
Bernie