did the third singer ever leave the group?
Yes, in 2001.
Siobhán Donaghy
Donaghy left the Sugababes in 2001 
what is 'push the button'? Song who is the writer of this song? Dallas Austin and the Sugababes. where was it recorded? UNANSWERABLE when did the song release? 26 September 2005 how much did the making of this song cost? UNANSWERABLE is sugababes the name of a group of singers? Yes, girl group. when did this group form? 1998 can you tell me the names of a few members from this group? Keisha Buchanan, Mutya Buena and Siobhán Donaghy. who is the lead singer of the song? UNANSWERABLE where is the first singer from? Westminster, London.
wiki:14552754
Siobhán Donaghy [SEP] Introduction
Siobhán Emma Donaghy (born 14 June 1984) is an English singer-songwriter. She is a founding member of the girl group Sugababes. Donaghy left the Sugababes in 2001 and released her debut solo album, "Revolution in Me", on 23 September 2003, which contained three single releases. Her second studio album, "Ghosts", was released on 25 June 2007. Donaghy appeared in a remixed version of the hit stage show "Rent" (titled "Rent Remixed") for its 2007–2008 run, playing the role of Mimi alongside Denise van Outen. On 20 July 2012, Donaghy and her former Sugababes colleagues, Buena and Buchanan, confirmed their reunion.