It depends on the song and the part in question. Sometimes they all sang all parts (e.g. the descending bit on 'Father to Son'), sometimes each took a part (e.g. 'Doing All Right'), sometimes only one of them sang every part (e.g. 'Love of My Life'), sometimes only two of them were involved (e.g. the wordless choir in the first verse of 'Killer Queen'), sometimes they all sang the middle bits but only Roger and/or Freddie sang the top and only Freddie sang the bottom (e.g. 'Somebody to Love').
For the first case, I reckon it was better for them to sing at the same time, because they wouldn't have to bounce that much (quality's lost a bit for each generation).
John hated Hot Space. Fred's favourite singer was not Paul Rodgers. Roger didn't compose 'Innuendo.' Witness testimonies are often inaccurate. Wales is not in England. 'Bo Rhap' hasn't got 180 voices.