Intresting!
So im guessing the HG612 is handing out the DHCP requested to the BT HH3 and also given all the WiFi/LAN clients a DHCP lease from the HG612 aswell (if that makes sense?)
I don't think that you'll be able to use 2 HH3's with one as a wireless repeater/extender with the stock/locked down firmware.
If the above info is right, There is a possible you can use as many as you wont
However! each HH3 need there gateway IP changed and also there DHCP server off so like, Example,
HH3 = 192.168.1.254 (Your main HH3)
HH3 = 192.168.1.250 (AP1)
HH3 = 192.168.1.249 (AP2)
etc etc but again is just a guess and this method you would pick up a DHCP request from the HG612 so no matter what BT HH3 your connected, you will always have the same LAN IP until that DHCP IP expires.