Complex sentence: Today the National Theatre consists of three artistic ensembles--opera, ballet and drama--which alternate in their performances in the historic building of the National Theatre, in the Theatre of the Estates and in the Kolowrat Theatre.
Split into simpler sentences: Today the National Theatre consists of three artistic ensembles: opera, ballet and drama. They alternate in their performances in the historic building of the National Theatre, in the Theatre of the Estates and in the Kolowrat Theatre.