Try these commands !!! It will work
set interfaces ge-x/x/x unit 0 family ethernet-switching port-mode trunk
set interfaces ge-x/x/x unit 0 family ethernet-switching vlan members WLAN_A
set interfaces ge-x/x/x unit 0 family ethernet-switching vlan members WLAN_B
set interfaces ge-x/x/x unit 0 family ethernet-switching native-vlan-id WLAN_C
Cheers !
Amit