Now days most of the softwares and New games don't run on XP Service pack 2 so, here is the superway to change windows XP service pack 2 to service pack 3 without any installationor formating.
Below are the simple steps.
Step 1: Run Regedit by Clicking on Start -> Run , type in regedit and press enter
Step 2: Make a backup of your registry (just in case)
Step 3: Browse to "HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\ Control\ Windows"
Step 4: Modify the value of "CSDVersion" from "0x00000200" (SP2) to the Windows XP SP3 value of "0x00000300"
Step 5: Close registry editor
Step 6: Reboot
Hope It Helps!!!