For clarification, I want to check if I've understood your post correctly. Please correct me if I'm wrong.
1. You have a character on OSRS.
2. Your RS3 character with the same name is a standard account.
3. You want to delete your RS3 character so that you can re-roll it as an ironman.
4. You do not want your OSRS character to be deleted.
I don't think it can be done. Jagex don't delete accounts. They disable accounts upon request, but this would not suit your purposes (you wouldn't be able to play your character on either game if your account was disabled).
I think you're going to have to roll a new alt to be your RS3 ironman.
Cranky old woman
04-Dec-2023 15:18:33