@KaneliXD If you've never registered your copy of Sims 3, you can do so now by using the product code included with the disc. It's the same code you'd have previously used when installing the game—it should be printed on the back of the insert that comes inside the case.
You can enter this code on the old Sims 3 site ( www.thesims3.com ) or within Origin. If the code reads as already used, is it possible that you already registered the game in either of these locations? If so, you'd just need to sign into Origin with the same email address and password you used when you originally registered the code.
Once your copy of Sims 3 shows up in your Origin game library, you can download it from there without needing to use the disc.