How do I enable a total of 3 BSSIDs for AP mode?

You must load Summit firmware for multi-BSSID support. After loading the driver and firmware, perform the following: Stop all software that may be managing the Wi-Fi interface including the supplicant, NetworkManager, connman, hostapd, etc.

List out the current virtual interfaces on the physical interface (phy):

# iw dev
phy#0
        Interface wlan0
                ifindex 7
                wdev 0x1
                addr c0:ee:40:50:00:00
                type managed
                txpower 20.00 dBm

 

Delete the virtual interface:

# iw wlan0 del

 

Add up to 3 virtual interfaces onto the phy using unique names per dev (note there are two underscores in __ap):

# iw phy0 interface add ap0 type __ap
# iw phy0 interface add ap1 type __ap
# iw phy0 interface add ap2 type __ap

 

Confirm the virtual interfaces exist:

# iw dev
phy#0
        Interface ap2
                ifindex 10
                wdev 0x4
                addr c0:ee:40:50:00:02
                type AP
                txpower 20.00 dBm
        Interface ap1
                ifindex 9
                wdev 0x3
                addr c0:ee:40:50:00:01
                type AP
                txpower 20.00 dBm
        Interface ap0
                ifindex 8
                wdev 0x2
                addr c0:ee:40:50:00:00
                type AP
                txpower 20.00 dBm

The MAC on the first interface will not change and each subsequent interface added will add 1 to the first MAC.