@Jon Kears Yes, you should now get three developer accounts with the sandbox. Are you the primary admin on the account? My dev accounts come up in the same place as sandbox etc.. when I hover over my name on the top right of any page

my CFO is the user with Internal ID -5, but he no longer has login access on the system. I think I would qualify as the primary admin according to Netsuite records but not sure if it's based on this internal ID

You could temporarily give access to that employee record and check, otherwise probably need to make a support case