Well, I'm no expert, but here's what I just did to install SP3 via ISOs to a base SLES 9 installation (not 64-bit, though).
1. Insert SLES 9 SP3 disk 1.
2. Run yast2 as root.
3. Choose Software->Change Source of Installation.
4. Choose Add, CD.
5. Highlight the entry just added, choose Up until it is at the top of the list, then choose Finish.
6. Choose Software->System Update.
7. Choose Next.
8. Swap disks as prompted.
9. Exist YaST when finished.
Hope that helps!