They have the potential to be....longevity will be the determining factor. Starc can be on and off, Haze is Mr Consistent while Pat Cummins is very, very special. He'll be the best of the three when their careers are done.
100% agree with you regarding Cummins. Very happy to see him come back from the horrible run of injuries he had during 2012-16 after his test debut, as it looked at one stage like his career was heading the direction of Jason Gillespie: lots of talent, but never quite reached his potential. It's going to be interesting to see if Cummins is captain after Tim Paine retires. He'll be the first bowler as Australian test captain since Richie Benaud
Hazlewood reminds me of McGrath: can bowl that nagging "corridor of uncertainty" (as Geoff Boycott would say) line and length that McGrath, Hadlee, Shaun Pollock and Ambrose were famous for. Cummins is just as accurate, but the difference is most of Cummins' deliveries have that extra bite off the pitch and I often wonder how different it would be if he got to bowl regularly with the new ball.
Starc in test cricket: hot and cold. When he is good, he is great to watch, but unfortunately has the tendency to spray the ball like a pinball machine.