I downloaded and installed the SP3 on my box at work this tuesday and here is what happened:
1. After the install it prompted me to reboot which I did.
2. Every time it booted from than on I got an error message that an entrypoint in a system dll was not found, however windows would load.
3. Wen. morning I started noticing some wierd things with my machine, for example I could not get the file sharing, or a handfull of other services to start even after trying manually to start them from the services snap-in.
4. I got sick of messing with it and ran an uninstall that afternoon (fortunetly that went without a hitch).
I have decided to stick with SP2 until they come out with another release, I would report this to MS however considering I just downloaded the beta SP off the web and am not part of the beta testing group I will just report to Anandtech!
-Spy