I have a single old Seagate 1tb, a 2tb WD black, and a 2tb WD blue. Should I RAID them and also if so which raid would be best for gaming as well as backup if possible. Oh and also a SSD but I don't think I'd include it in the raid.
I would not recommend RAID. You will be much better off using the disks for BACKUPS rather than RAID. There are many types of failures that are not protected by RAID.
It doesn't matter if the disk is internal or external. The important thing is to use software that will automate backups. "Backups" are no good if you don't automate them and have them happen every day (or week or whatever).