The second building is two times as tall as the first building, a total of 600*2 = <<600*2=1200>>1200 feet.
The total height of the first and the second building is 1200+600 = <<1200+600=1800>>1800 feet.
The third building is three times as tall as the combined height of the first and second buildings, which is 1800*3 = <<1800*3=5400>>5400 feet.
Together, the three buildings have a total height of 5400+1800 = <<5400+1800=7200>>7200 feet.
#### 7200