Seatrain Lines Inc., the shipping and chartering firm with mostly red ink in its past and corporate defections in its present, today was forced into bankruptcy by three Italian companies that claim Seatrain owes them more than $1.1 million.

That sum, however, is much less than the estimated $60 million Seatrain owes Chase Manhattan Bank, its principal lender. But Chase was not a party to today's petition, and a spokesman for the bank said Chase was "disappointed with the action. It came at a time when every effort is being made" to bring the Seatrain back to profitability.

Apparently the way back appeared too fraught with obstacles for the three Italian firms that provided Seatrain with transportation repairs and similar services in Genoa and other cities. This morning, in a petition filed in U.S. District Court here, they asked the court to put Seatrain into involuntary bankruptcy.

This afternoon Seatrain Chairman Howard M. Pack filed a "consent" to the bankruptcy filing, and the company said it will continue to operate as a "debtor in possession" while it tries to come up with a reorganization plan under court auspices.

The Chase spokesman said the bank believes "Seatrain will continue to operate under order and emerge with a viable plan of reorganization."

Seatrain, which lost $33.9 million in its fiscal 1980 (which ended last June 30), estimated last week that it would lost $150 million in the second quarter that ended Dec. 31. At the same time, four of the company's nine directors, including President Stephen Russell, left Seatrain.

Seatrain attributed the huge second-quarter loss (it made $3 million during the last three months of 1979) to costs stemming from the closedown of its New York shipyard in May 1979, charges from the sale last December of its Pacific container shipping operation, high interest rates and some costs of uncompleted ships.

Last September, when Seatrain reported a first-quarter loss of $4.9 million, it estimated that closing the shipyard, on the site of the old Brooklyn Navy Yard, would cost $20.6 million. Company officials now say the costs will be higher.

They did not return a reporter's phone calls today.

Chase Manhattan and other Seatrain creditors have been nervous about the company for a long time -- it has made a profit in only three of the last 10 years and has not paid a dividend to investors in more than two decades -- but the bank has decided several times in the last few months to continue to allow the company to operate without pressing for repayment of the loans.