Question: Which band along with two other British bands is referred to as the "unholy trinity of British hard rock and heavy metal in the early to mid-seventies" whose performance was featured in a 2011 studio compilation album at the BBC's Maida Vale Studios?
Related documents: [0]  [DOC] Deep Purple are an English rock band formed in Hertford in 1968. The band is considered to be among the pioneers of heavy metal and modern hard rock, although their musical approach changed over the years. Originally formed as a progressive rock band, the band shifted to a heavier sound in 1970. Deep Purple, together with Led Zeppelin and Black Sabbath, have been referred to as the "unholy trinity of British hard rock and heavy metal in the early to mid-seventies". They were listed in the 1975 "Guinness Book of World Records" as "the globe's loudest band" for a 1972 concert at London's Rainbow Theatre, and have sold over 100 million albums worldwide. [END] 
[1]  [DOC] BBC Sessions 1968–1970 is a 2011 studio compilation album featuring performances by the British hard rock band Deep Purple that were recorded at the BBC's Maida Vale Studios, London, and originally broadcast on various BBC Radio shows from 1968 through 1970. "BBC Sessions 1968–1970" is a two-disc set collecting all the surviving sessions in the BBC archives. [END] 
Answer: 
Deep Purple
Deep Purple