The event invite is a bit vague, of course; there's no mention of the Galaxy S III specifically, just a "come and meet the next Galaxy" strap line. There aren't many other Galaxy models Samsung would use an evening event in London to launch, apart from a flagship device.
If it's launching on May 3rd, we could expect it to hit the shops pretty soon after. It looks like those rumours of a late April launch weren't all that far off after all. More here.