There is a SP3 for XP OFFICE but not for XP its self.
The following link from Microsoft says it is planned for 2007 http://www.microsoft.com/windows/lifecycle/servicepacks.mspx
I hope that answers your question.
I've been told that SP 3 has already been made and able to download.
Is that true? I haven't found any SP 3 on microsoft official website.