TORONTO, JUNE 5 -- Prime Minister Brian Mulroney and Canada's 10 provincial premiers continued their marathon meetings today in an effort to salvage proposed constitutional amendments aimed at dampening separatist sentiment in French-speaking Quebec and averting breakup of the 123-year-old Canadian confederation.

After nearly 12 hours of negotiating on this third day of the talks, the premiers emerged at 9:30 p.m. sounding more confident than before about a possible resolution to the constitutional impasse but stressing that no conclusive agreements had been made and that any consensus reached was only tentative and contingent on future agreement on a number of issues. They are to meet again Wednesday.

Mulroney, in a nationally televised statement, said that some progress had been made, but quickly added, "We have a lot of work. We're a long way from any final arrangements."

With pressure mounting on the holdout premiers of Newfoundland and Manitoba to join their colleagues in approving the constitutional proposals -- known collectively as the Meech Lake accord -- the focus of the impasse continued to shift from special status for Quebec as a "distinct society" to disputes over how Canada's appointed and mostly powerless Senate should be reformed.

In 1986, Quebec sought and later won in the Meech Lake accord a requirement for provincial unanimity on any reform of the Senate, but some provincial leaders now say that such a formula would make it virtually impossible to alter the powers of the upper house of Parliament.

Monday night, a third Meech Lake holdout, New Brunswick Premier Frank McKenna, indicated he was prepared to approve the constitutional amendments, which were initialed by Mulroney and the premiers at a Quebec fishing lodge three years ago. Following that agreement, however, governments changed in New Brunswick, Newfoundland and Manitoba, and all three rescinded or refused to ratify the accord, which is set to expire June 23 unless formally approved by all 10 provincial legislatures.

Newfoundland Premier Clyde Wells has shown little sign of relaxing his opposition to the accord as it now stands, telling reporters this morning, "I cannot and will not ask the legislature of Newfoundland to pass Meech Lake as it is."

Gary Filmon, premier of Manitoba, also has opposed allowing a single province to have a veto over Senate reform, saying that nothing will be accomplished if unanimity is required. Under existing constitutional provisions, seven provinces comprising half the country's population are needed to reform the Senate.

Quebec's legislature, reflecting a growing movement among French Canadians for political sovereignty with economic ties to English-speaking Canada, has declared it will not approve the Meech Lake accord if it is changed. Polls have shown that more than 60 percent of the residents of Quebec favor political autonomy if the accord collapses.

The Canadian Broadcasting Corp. reported that Mulroney and the provincial leaders had reached a "fragile agreement," contingent on other issues being resolved, that a national commission on Senate reform would be named to hold hearings and make recommendations.

If the commission failed to agree on reforms, the present Senate would be expanded to increase the seats from the smaller provinces to 10 from six, while the populous provinces of Ontario and Quebec would retain 24 each.

Quebec Premier Robert Bourassa said he tentatively approves of the proposal to revamp the Senate later, as long as the original Meech Lake accord is adopted unaltered -- still a sticking point with Newfoundland and Manitoba.