Complex sentence: Universal Music Group, Sony Music Entertainment and Warner Music Group; formerly the Big Six, but Universal acquired PolyGram in 1998 and merged with EMI in 2012, and Sony and BMG merged in 2004.
Split into simpler sentences: Universal Music Group, Sony Music Entertainment and Warner Music Group. Formerly the Big Six: before Universal acquired PolyGram in 1998 and merged with EMI in 2012, and Sony and BMG merged in 2004.