|A frequent reason why the SP3 Updates don't install is you have one or more resident modules of anti-malware software running that is / are interfering with it's proper installation. |
In addition to the following, they DO interfere with the proper installation of SP3 updates
NOTE that sometimes the resident module(s) of anti-malware programs - a part that runs all the time scanning for suspicious activity - will interfere with the proper installation of third party software, the software will not install properly, and you may get no indication of that at all while installing the software.
To avoid that possibilty, you should always DISABLE the resident module(s) of anti-malware programs, BEFORE you install third party software (software other than most Microsoft Updates, etc., that did not come with Windows ), especially when it's a major or complicated software package.
E.g. if you are using the free or paid version of AVG, you should disable the Resident Shield in AVG's 's settings in Windows. In Norton (Symantec) products, there may be several things you need to disable, or set so they don't load for a specific short amount of time.
If you don't know how to do that, tell us which anti-malware software you are using.
NOTE that Symantec (Norton) products are best UN-INSTALLED, BEFORE you install SP3 updates, because you can't disable their resident modules for more than a short time.