You can download and install either vBulletin 3.x or 4.x with your license. However I would strongly suggest you download 4.x, as 3.x is very old and hasn't bee developed for years now.
There is a "Vbulletin 3" style for VB4 available here:
https://vborg.vbsupport.ru/showthread.php?t=275621
This is a VB4 Style that looks a lot like VB3 used to. You cannot use actual VB3 styles in VB4, they are incompatible.