Transatlantic is already in discussions with Validus Holdings Ltd, Berkshire Hathaway unit National Indemnity Co and an additional undisclosed party.
Last month, it was reported that a former Berkshire insurance executive, Joseph Brandon, was working with Morgan Stanley on a possible bid for Transatlantic and was the undisclosed party in question.
Transatlantic had a merger agreement with its peer Allied World, which was struck in June, but both sides called off the deal in September amid overwhelming opposition.
Validus, which has been interested in Transatlantic for years, launched a hostile bid in July but agreed to a limited standstill while in talks with the company.