When trying to activate an ArcGIS Desktop extension license in either ArcMap or ArcCatalog, the following error is displayed:
Error: Extensions The extension could not be activated. License request failed. All 3D Analyst licenses are in use.
Note: This article pertains to ArcGIS versions 9.x. Later versions of ArcGIS may contain different functionality, as well as different names and locations for menus, commands and geoprocessing tools.
All available licenses for that particular ArcGIS Desktop extension are in use.
To work around this issue: