Hi Sarah
As far as I know, a child born out of wedlock is always given the mother's surname unless the father admits paternity. Nowadays, the father would actually have to be present at the registration for his name to appear on the birth certificate.
Frederick George Stapleton bn in 1868, parents i believe are Charles Walker and Mary Ann Stapleton married 1871. This is the one that causes me the most confusion, as Frederick went on to use the name Walker.
Is it likely that Charles was his father even though they didnt marry till 3 years later?
If Charles was Frederick's father why would they have waited 3 years before getting married
If Mary Ann was unmarried (to anyone) when Frederick was born, he would be registered as Stapleton. The birth certificate may well not show a father's name. Unless there is a name, you will have no way of knowing if Charles was his father
When Mary Ann married Charles he obviously took on Frederick as his step-son. As he was only 3, it was probably easier to use Charles' surname for Frederick, especially if more children were born later.
I have seen instances where children continue to use their birth surnames, even though they are part of the new family
If Charles really was Frederick's father, he may already have been married to someone else, so he and Mary Ann had to wait until his first wife died or they were divorced (much less likely in the late 1800s)
If you find a marriage certificate for Frederick it is likely to show Charles as his father. This is not proof that he was the birth father, just that he was the man Frederick knew as Dad!

Linda