We use R600 unleashed APs for dedicated wireless requirements during corporate events and have never seen a problem like this before. However saying this, I wouldn't recommend that you ran 400 users over just 3 APs. Although they are within scope, their throughput would be subject to a bottleneck experience.
I assume that they are all connected to the same VLAN and route traffic through a shared gateway? Are the APs connected at 1Gbps from the switch? Have you added a second LAG link per AP? Is there any rate limiting from the switches or APs?