We just went through the installation of Service Pack 3 and Update Rollup 2 and thought it would be a good idea to pass along some helpful information if you're planning on deploying them in your environment.
As always, you'll want to test them out in your test environment. Don't have one? We didn't either so we did a lot of reading to try and spot any potential problems before they came up (see the links below). Next, you'll want to make sure you have a full backup of your server(s). Remember that if you're running Exchange on VM's, taking a snapshot isn't a viable supported option. You want to make sure you're using backup software that does an OS and application level backup, as well as a proper Exchange database backup.
Feeling confident? Go ahead and get the bits for the install:
Exchange 2010 SP3
Update Rollup 2
If you have multiple servers in your environment, you want to start installing from the outside in, meaning Edge servers, then CAS HUB's, and finally mailbox servers. Just a head's up, that people will tell you it takes about 30 - 40 minutes to install per server. Not the case in our environment. It took roughly two hours per box to install both updates and Windows Updates. As always, there are a lot of reboots.
Before you run either update on a server, be sure to stop your antivirus software. Also, if you use DAG's for your mailbox servers, you need to install the hotfix from KB 2550886.
Hope this helps, and good luck. Let us know if you find any other gotcha's.