Thank you for following up. This issue is resolved. I can't explain the resolution other than describe the process that worked.

I restarted the SP3 install, opened a window to SEARCH the CD drive, found SKU1CA.cab in the S section, went back to the SP3 installation dialog box and it continued without asking for SKU1CA.cab.

I don't know why SEARCH the CD drive would be different than EXPLORE (see my first post) but it worked.

SEARCH was the only thing I did different than my first installation attempt.