Is there a singles category or a sub category for finishing just one of the games? Like if I beat the original super mario, can I submit that run somewhere?

You can submit individual game runs to the original game leaderboards which have All-Stars categories with the exception of SMB3 which combines both versions into the same leaderboard.

https://www.speedrun.com/smb1 https://www.speedrun.com/smbtll https://www.speedrun.com/smb2 https://www.speedrun.com/smb3

Oh I didn’t know that at all. I thought they were two separate games, which in a way they are. Thank you so much.