MS seems to have removed or broken the CFS3 update patch pages.
You don't need this anyway with most recent CDs or DVD versions of CFS3 but in case you have an old one;
You can get 3.1 patch from here:
Combat Flight Simulator 3 v3.1 Patch
cfs3updateeng.exe | 4.17 MB (English)http://www.gamershell.com/download_1257.shtmlhttp://www.fileplanet.com/121232/120000/fileinfo/Combat-Flight-Simulator-3-v3.1-Patch
or get the appropriate language version, French, German, Italian, Spanish is also listed here:http://www.gamefront.com/files/listing/pub2/combat-flight-simulator-3
3.1a is not necessary for OFF but is available from the link above:
CFS 3 B-25H/J aircraft Fix for v3.1
acupdate.exe | 171.08 KB
In future we'll try to make them available on our website.