Jopalli Krishna Rao, Dr Rajaiah and S Sathyanarayana tendered their resignations to Andhra Pradesh Congress chief B Sathyanarayana.
The three MLAs officially joined the TRS at a programme in Hyderabad at 4 pm.
The MLAs, when contacted, said they were tired of their party's inconclusive stand on the Telangana issue.
Krishna Rao said that never in the history of India had such a protest taken place.
So many employees are on strike and despite that the Congress has not acted, he said.
TRS General Secretary Subhash Reddy said, "We are expecting 6 more MLAs and MPs from the Congress to join us soon. This time around the resignation has been given to the party president unlike the last time when it was given to the Speaker. They cannot sit over it or reject it this time."