The specs say it has one 10/100 Ethernet port, so surely it's just a modem. Perhaps that 10/100 port is the reason it only supports up to 100Mbps.
If using with Three I'm not sure it will work, doesn't Three use 800 Mhz frequency? Note, I could well be wrong on this.
View my FTTC cabinet map for Ramsgate, Broadstairs, Margate and Westgate
Roadworks.org - the place to see what BT are up to - not all counties are covered
My Photobucket guide to installing a xDSL extension for a modem away from the master socket