For most people there is no problem with SP3.
There are a few specific instances such as some HP with AMD processors which were loaded with the same drive image as the Intel processors thus having an incorrect processor management file which when replaced causes problems on reboot.
But generally speaking if your machine is clean of malware, and otherwise up to date , sp3 is a fairly minor set of changes.
As always, you should backup vital data, you should do a basic disk cleanup (empty temp and temp internet files, and recycle bin ; run error checking and defrag the disk ) before you install the service pack. Then make sure you have a clean restore point too just in case.