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?

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. <SEP> Over the years, the British hard rock band Uriah Heep has released 24 studio albums, 13 live albums, 16 compilation albums, 27 UK singles (33 worldwide) and 17 videos.  The band's best selling album is "Sweet Freedom" which was released in 1972 and its worldwide sales are more than 4 million copies.  Uriah Heep's progressive/art rock/heavy metal fusion's distinctive features have always featured massive keyboards sound, strong vocal harmonies and (in the early years) David Byron's operatic vocals.  Twelve of the band's albums have made it to the UK Albums Chart ("Return to Fantasy" reached No. 7 in 1975) while of the fifteen "Billboard" 200 Uriah Heep albums "Demons and Wizards" was the most successful (#23, 1972).  In the late 1970s the band had massive success in Germany, where the "Lady in Black" single was a big hit.  With Black Sabbath, Deep Purple and Led Zeppelin, Uriah Heep became one of the top heavy metal and hard rock bands of the 1970s.  It was there that the solid, but rather mainstream-sounding "Sweet Freedom" (No. 18 UK, No. 33 USA) was created with "Stealin'" released as a single.  Having gained worldwide recognition, the band quit using fantasy lyrics and made an obvious stab at versatility by adding funk ("Dreamer") and acoustic folk ("Circus") elements to the palette. <SEP> The discography of British singer-songwriter Gabrielle Aplin, consists of two studio albums, four extended plays, three live albums and five singles.  Her first release was the 5-track "Acoustic EP" which was released on the iTunes Store on 13 September 2010.  Her second EP "Never Fade" was released on 9 May 2011 and saw Aplin expand her sound, showcasing a more folk rock sound and playing all instruments herself.  In April 2011, Aplin was invited to perform for "BBC Introducing" at Maida Vale Studios, where she played 3 tracks from "Never Fade" and a cover of the Coldplay song "Fix You".  Aplin released her third EP, "Home", on 9 January 2012.  On 29 February 2012, Aplin announced that she had signed to Parlophone.  Aplin was confirmed as the soundtrack to the John Lewis 2012 Christmas television advertisement, covering Frankie Goes to Hollywood's "The Power of Love", the song reached number one on the UK Singles Chart.  On 12 December 2012, Aplin announced that the title of her debut album would be "English Rain".  In addition, she also unveiled its artwork and release date of 29 April 2013.  However, the album's release date was later confirmed as 13 May 2013.  Aplin announced live on 17 February Radio 1 Chart Show that her third single would be "Panic Cord".  The song originally featured on her Never Fade EP and it was released on 5 May 2013, charting at number 19 on the UK Singles Chart.  "English Rain" charted at number 2 on both the UK Albums Chart and Scottish Albums Chart, while reaching number on the Irish Albums Chart.  In 2014, Aplin released her "English Rain EP" in the United States.  The EP was released on the 6th of May and features 5 songs from her debut album, as well as a cover of Canadian singer Joni Mitchell's "A Case of You".  In 2015, Aplin released her second studio album entitled "Light Up the Dark".  "Light Up the Dark" debuted at number 14 on the UK Albums Chart. <SEP> 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.  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.  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".