As far as I know this is not possible using Linux bridging; you can only add an interface (eth0, wlan0, mic0) to a bridge once.
I would recommend against such a setup anyway , as it will kill (network) performance of your Phi's completely - a "normal" mic interface has an MTU of 64512 whereas a bridged interface would have an MTU of 1500 (or possibly 9000 if your network does jumbo frames). Not only does the smaller MTU size kill performance, also remember that in a bridge *ALL* traffic entering one bridged adapter is copied out to *ALL* other adapters - if you have a chatty network you'd be flooding the MICs with useless packets.
