A few comments--
For 2, you could use Gemini, Thomas, Cowen, or something meaning twin.
For 3, it's common in the southern US for Billy Bob III to be called Tre/Trey.
Roman family have the -gens or family name and then add the number to it-- so Julia Maior and Julia Minor (elder and younger) or Julia Tertia (third). Some Korean families I know do this where the names they call each within the family are "older sister" or "youngest" or "middle."
I can list some more number names when I find my Oxford Latin Dictionary but I learned last year in class that in most Roman families the boys would get individual names and girls would be named after their father. A man named Iulius (no J's in Latin) would have Iulia Prima, Iulia Secunda, Iulia Tertia, etc.