There is a problem with this in WM3-SP1 or older version of WM.
We have fixed this issue in WM3 Service Pack 2 patch. The fix will allow the WM to start while setting the exceeding devices to unmanaged state.
To resolve your problem please download and install the WM3 Service pack 2 or newer patch to your WM server. You can download them from this page: http://www.cambiumnetworks.com/support/management-tools/wireless-manager/
Service packs are installed using the UpdateManager.sh You will need to exit and start it again after each pack is installed (SP1, SP2 and then SP3). The SP will prevent this "out of licenses" issue from happening to you in the future.
Alternatively, here is a WORKAROUND you can allow the WM to start even without the patch by setting some devices to unmanaged state using MySQL (provided that you are familiar with MySQL console). Here are some sample SQL queries that will help you:
use WMSDB;
select count(*) from ManagedObject WHERE MANAGED='true';
//This will return number of devices that consume a license
select * from ManagedObject WHERE TYPE='Generic Element' AND MANAGED='true';
//This will list all the Generic Elements that are consuming licenses
update ManagedObject SET MANAGED='False' where TYPE='Generic Element' and MANAGED='true';
//This will set all Generic Elements to unmanaged state (in case that the previous query returned some results)