I keep hearing people say 'I watched black butler season 3 first episode' and I want to know if there is a legal site online where I can watch Episode 1.
Thanks to @アズーサ, Hulu is streaming Season 3
You can also watch it on Funimation's Site
CrunchyRoll have also got the series for streaming. (Without English subtitles for the moment however)
Season 3 or "Book of Circus" has only started releasing (July 11, 2014) at the time of writing, so there's no physical product to buy. Perhaps someone will pick it up once the season is over.