clients will be connecting to VLAN3. WLAN overrides what WLAN needs to be enabled/broadcasted on specific AP while AP group overrides WLAN to VLAN mappings.
http://www.cisco.com/en/US/docs/wireless/controller/4.2/configuration/guide/c42wlan.html#wp1127323
Configuring WLAN Override
By default, access points transmit all defined WLANs on the controller. However, you can use the WLAN override option to select which WLANs are transmitted and which are not on a per access point basis. For example, you can use WLAN override to control where in the network the guest WLAN is transmitted, or you can use it to disable a specific WLAN in a certain area of the network.
Configuring Access Point Groups
In a typical deployment, all users on a WLAN are mapped to a single interface on the controller. Therefore, all users associated with that WLAN are on the same subnet or VLAN. However, you can override this default WLAN setting to distribute the load among several interfaces or to group users based on specific criteria such as individual departments (for example, marketing) by creating access point groups (formerly known as site-specific VLANs). Additionally, these access point groups can be configured in separate VLANs to simplify network administration
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_configuration_example09186a008073c723.shtml
AP Group VLANs with Wireless LAN Controllers Configuration Example
