Ananta Deb Adhikary and Dasarath Tirkey of the Revolutionary Socialist Party and Sunil Mondal of the Forward Bloc formally pitched their lot with the Trinamool in the presence of its national general secretary Mukul Roy.
The troika had cross-voted for Trinamool candidate Ahmed Hasan Imran in the elections to the upper house Friday.
Roy said two Congress members - Sushil Roy and Imani Biswas - who had also cross-voted for Imran, were also ready to join the Trinamool.
"The three Left Front legislators have joined us...they had wanted to join us and they wanted to quit as legislators. As part of the political strategy...they will resign when the time is right but from today they have joined us...we did not let them resign beforehand due to political reasons," Roy told the media.
"The three cast their votes for TMC to protest against the activities of the Left parties. The other two...Congress MLAs are ready to join us.
"You will notice the five legislators belong to the Scheduled Castes, Scheduled Tribe and minorities," said Roy.
He claimed "plenty" of Communist Party of India-Marxist members have contacted the Trinamool to join its fold.
"Due to the bad policies of the Left Front, and particularly the CPI-M, several of its members are on the fringe...and may move over to TMC. The way CPI-M leaders are running their party ...they should look inwards," he warned the opposition.
He asserted CPI-M is "inconsequential" at the moment.
"For 35 years they had indulged in hooliganism and were finally ousted by the public...they will not be seen for the next few decades..they have been wiped out," he said.