SONY

GameSpot: http://e3.gamespot.com/press-conference/sony-e3/
GameTrailers: http://www.gametrailers.com/e3
IGN: http://www.ign.com/events/e3
Ustream: http://www.ustream.tv/PlayStation
Twitch: http://www.twitch.tv/Twitch

MICROSOFT

Xbox.com: http://www.xbox.com/en-US/e3#fbid=Hech-OXFVgg
On your Xbox
On Spike TV
Or with the Windows Phone app
GameTrailers: http://www.gametrailers.com/e3
Twitch: http://www.twitch.tv/Twitch

NINTENDO 

US: http://e3.nintendo.com
JP: http://live.nicovideo.jp/watch/lv181562623
EU: https://www.nintendo.co.uk/Games/E3/…14-891073.html
YouTube (JP): http://www.youtube.com/watch?v=TeBmaBBQqvM
Twitch: http://www.twitch.tv/Twitch

Source