The first rooftop is at the building just south of the northern-most Handsome Jack statue. To climb it, just walk up the sloped side of the building to the balcony at the top.
To reach the second rooftop, the Vault Hunter will have to climb a huge beam in front of the vending stations at the southwest shore, near the bridge leading to the huge crane. A BUL loader will spawn upon reaching the rooftop.
The last rooftop can only be reached via the window washer platform at the southwest side of the office building opposite of the helipad in west Opportunity. To reach it, a second Vault Hunter is required to press the elevator button after climbing the platform via the support beam below. It is possible to reach the rooftop by oneself (except for Krieg) by combining a grenade jump and a rocket jump from the Badaboom while under the drunken effect of the Grog Nozzle in order to land on the window washer platform in its upper position.