Basically they are the only teams that could win all 3 European trophies. Man yoo need to win the eufa to complete all 3, but wouldn't want to be in the comp as it would mean they are not in the chumps league, Arsenal nearly did it last year, City and Everton have both won the cup winners cup, so could do it!!
Liverpool and Celtic have never won the Cup Winners Cup and as it has now been binned can never complete the treble.