Currently you will receive Zero Security Updates on XP SP2, MS support (KB, security updates) for XP SP2 ended over 18 months ago (I can't recall the exact date). So it won't offer any updates much less SP3, you will have to manually download it, install it and then windows update should offer the KBs released since SP3, be ready for a flood.
SP3 full download is in the order of 363MB if I remember rightly, I had to get someone on broadband download that for me (when I was on dial-up). Even after that, the number of KBs would tax anyone on dial-up and manually downloading them (for off-line installation) would not be an easy task.