<<My Genuine OS Disk is SP2, and I have the SP3 disk that was supplied along with a Recovery Disk. It is a genuine Full Retail version.>>
I guess that I don't understand how...you can have a Recovery Disk, along with a MS Genuine XP CD...and whatever your "SP3 disk" is. Reocvery disks are issued by OEM system manufacturers, in lieu of providing a legal copy of Windows. Please clarify exactly what each disk might be.
<<My question about running SFC.exe is will it work with the SP2 installation disk, even though I have SP3 installed?>>
Such is not advised.
You can update your SP2 CD, using a program like nLite. That created CD then replaces your original install disk.
nLite - Deployment Tool for the bootable Unattended Windows installation - Guide - http://www.nliteos.com/guide/