[Ashen]

Aha, no problem, I've closed three of this kind already.

What you do is get on top of the two you can access and flip the switches, breaks, that are on top of each. This will extend a bridge from these two towers onto the third (and possibly between these two but this is of just one use, look below).

Now if you are on top when you flip the second switch what you need to do is get back lower till about the middle of the tower, you need to be in the central chamber. There will be doors here marked - the Wastes of Oblivion - (apart from those leading upwards) open them and you'll find yourself facing the bridge to the central tower.

You can do something else too, to help you out. While you are on top, go down just a little bit lower, where there is that giant red circle thingie, just below the switch. There in the corridor there is another one of these gates to Oblivion plane. Now those lead to the tower where the first switch is, but they are handy in the way that they will show you, from the outside, where the bridge to the third tower is, how much lower, so you can judge how far down you need to go. But it shouldn't be a problem as I've said, considering that at only that point below, in the central chamber, is another door to the plane of Oblivion.