Release varies depending on where you are (Europe/America), although seeing as the PC version will be released digitally there is a chance you will be able to download it on the earlier European release. So if that’s the case it might be available for download on the 29th May – although the release dates are so vague it’s hard to tell.
Multiplayer might not be in the PC version, the quote is… ‘Take your career online and compete for the ultimate title of ‘Virtua Tennis Champion’ with the new PS3 and Xbox 360 online ranking system.’ This suggests that there won’t be a ranking system for the other versions of the game, if the other versions will get online or not is hard to tell, there has never been any confirmation as far as I know. In my view VT3 should have had online play for the PC and this game should for sure.
As far as I know the requirements have not been released yet.