It will be released on March the 29th, if you have already bought season 1 and 2 on the Xbox One you will get access to those on Windows 10 for free as it will be cross buy so then all you need to do is purchase season 3, though there has been no word on pricing yet.
There will be four characters at launch with an additional four characters to be released later and three new stages, the four launch characters are Kim Wu and Tusk both returning from Killer Instinct 2, Rash from the Battletoads game and Arbiter from the Halo series.
In addition there will be changes to all the characters as well as a new lighting system and gameplay adjustments.
Here is the minimum and recommend specs for the Windows 10 version.
My own thoughts: It is no secret that Killer Instinct is one of my favourite fighting games ever and with season 3 it just makes the game better, all i ask is make the ultras look a bit better ( I even had a go and made a ultra combo lens flare video on Youtube) and perhaps make the hit counter font get crazier when the hits go up.