Theo
If your forums are quite critical you should try everything first on a test server.
Do a clean instal of 5.1.2 on test machine (can be a low spec machine), apply the same updates & rpms as your production server then backup the data and config from production server and restore on your test server (perhaps using backup/restore to/from desktop if you do not have a tape drive).
Test everything is running OK then apply update 3 and test again.
If OK go ahead and apply to your production system.
Keep the test system for next time you want to do some update or other and prove it first.
Regards
Ray Mitchell