There are plenty of excellent sources of help here but they are going to need more info to help you out. Questions that came to mind as I read this include:
1) Are you running autonomous APs? (code version(s)?)
2) Are you running LWAPP APs? (controller type(s) & Code version(s)?)
3) Bandwidth requirements of the DB app? If an application happens to have high bandwidth demands, then wireless may be seen as the cause of the problem when in truth you are working with a smaller bottleneck. And poorly written network applications tend to work even more poorly with a smaller bottleneck.
4) It is noticibly slow when 3 users are accessing the system over wireless (how about when it is just 3 wired users?). Is there some background processing that ocurs when users are idle?
5) Are you using QoS for Voice over wireless or anything else that might take precedence over this wireless DB traffic?
That should give us a little more to work with, thanks.