Connecting 3 LAN sites with Dlink 900AP+. Help please.

This is a long LONG saga but I'll keep it short.

My current question is what mode and gateway should I set for the wireless
AP's.

I have 2 Dlink900AP+ connecting SiteA to SiteB (400m)
I have 2 Dlink900AP+ connecting SiteB to SiteC (About 4km)
The connections are pretty good
Both AP's at siteB are connected to the ethernet LAN
The main aim is to get SiteC to reach it's PDC at SiteA
I'm going loopy trying to get the settings right.
I will post this hoping for a quick responce whilst typing a more detailed
post.