Phil Dwyer, who holds the record for most league games for the club, was allocated membership number 1, with Jason Perry No 2 and Andy Legg No 3.
They joined up at a special night in Cardiff's Electricity Club, organised to mark the official public launch of the trust, which was attended by around 300 City fans.
And there was a rush of supporters wanting to become part of the trust with 172 signing up on the night for one of the three types of membership, which has already topped 200.
Acting Chairman Paul Corkrey said: "We're delighted that three City greats like Phil Dwyer, Jason Perry and Andy Legg not only came along to the launch but accepted our invitation to become the first three members of the trust.
"We had a great night which was really well backed by fans, who like us believe that there is need for Cardiff City supporters to have a strong and independent voice.
"We're delighted that so many supporters also signed up to become members of the trust and I would encourage all City fans to follow suit. The more members that sign up the greater the voice we have on behalf of the fans. Now the launch is over, the hard work starts."
Further information on Cardiff City Supporters Trust can be obtained by logging on to www.ccfctrust.org where a membership and standing order forms can also be downloaded.