Joseph V. Ossorio, the former chairman of Drysdale Securities Corp. and a former director of Drysdale Government Securities Inc., pleaded guilty Wednesday in U.S. District Court here to three counts of securities fraud.

Drysdale Government Securities, in business only from February to May of 1982, was a spin-off of the 92-year-old Drysdale Securities Corp. now in liquidation.

U.S. Attorney John S. Martin Jr. stated that the criminal charges resulting in Ossorio's guilty plea were the result of a major investigation into the trading practices of Drysdale Securities Corp. and Drysdale Government Securities, practices that culminated in May 1982 in approximately $300 million in losses to Chase Manhattan Bank, Manufacturers Hanover Trust Co. and others. Martin said the investigation is continuing.

Andrew J. Levander, the prosecutor in charge of the case, stated in court that Ossorio's guilty plea "is essentially unrelated" to the collapse of Drysdale Government Securities Inc.

Levander said further: "Although Ossorio has significant knowledge and information about those events, his statements to the U.S. attorney's office . . . reflect that he is not criminally liable for the frauds that led to the collapse of Drysdale Government Securities Inc."

Ossorio, 42, of Greenwich, Conn., entered his guilty plea before U.S. District Judge Morris E. Lasker, who set June 30 as the tentative date for sentencing. The government asked to leave the date open, indicating that Ossorio is cooperating with the government.

The charges in the three-count criminal information grew out of Ossorio's management and reporting regarding more than 15 securities-investment accounts.

According to that document, these accounts were established in the name of various offshore corporations in the Caribbean. The information charges that between 1976 and June 1982, Ossorio's trades caused the offshore accounts to lose more than $5 million.