To reach the first comms tower go through to the back of the building it is on top of. Follow the walkway around and jump up to the roof to get to the first comms tower:
To get to the second comms tower follow the rooftops to it. Here you have to deactivate an electric fence, see the following guide for how to do that: http://wp.me/p1SfP0-1PN
To reach the third comms tower go back to the electrical fuse box you used to shut down the electric fence. Use the elevated part of the roof to jump up to the top roof. Now you can get to the third comms tower.