2000 users in a 300-600mtr range? That sounds like a market place or festival or congress centre.
I see you are aiming at mobile users with 2,4Ghz radio's (smart phones, tablets, laptops etc.) Since 2,4Ghz is the standard here this is where you have to go for.
But since 200 devices in reach of AP (NOT using traffic, just associated) already stretches most hardware to its limit we still talk at least 10 AP's.
Since you only have 3 none-overlapping channels available AND you need more AP's anyway you have to start working with several small cells.
Best to work now is with omni antennas and not too big a cell to reduce interferences as much as possible.
But now you need a network on top of these AP's to server them and connect them back to the main internet connection. This has to be build in 5Ghz.
The whole picture you are giving us is basically very complicated and as a result of that you can forget to build the 'poor man's internet'. It's going to be a expensive project...
The only other way you can try is indeed to setup one 3 sector AP that gives reach to your zone of interest, but never expect more than 30-50 competitive users at the same time.
Don't expect too much signal penetration inside buildings. The use of the Airmax antenna (I am not a fan of these, there promise much more than they deliver...) implicates they have to be relative low 'shooting over its users' but that immediately degreases their angle to shoot into buildings from 'above'.
Don't think too light about all this. A failure and loss of money is just around the corner....
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Rudy R. Puister
WISP operator based on MT routerboard & ROS.