n a major Cabinet reshuffle, British Prime Minister Margaret Thatcher today fired three leading government members who were critical of her controversial monetary policies and transferred a fourth to Northern Ireland.

Her moves were a clear indication of the hard-line Conservative government's determination to reduce inflation in Britain through tight money policies. Some 3 million Britons are out of work, about 11 percent.

In a new sign of the increasing unpopularity of her government, an opinion poll published today indicated that a general election now would be won by an alliance of the new Social Democratic Party and the Liberals. The poll clearly showed voter disapproval of Thatcher's policies and also of the increasingly leftward swing of the opposition Labor Party.

The Social Democrats, a middle-of-the-road group which broke away from the Labor Party, is trying to occupy the center of British politics alongside the small Liberal Party.

Thatcher dismissed her deputy foreign minister, Sir Ian Gilmour, her education minister, Mark Carlisle, and Lord Soames, majority leader of the House of Lords. Employment Secretary James Prior was transferred against his wishes to become secretary of state for Northern Ireland.

Lord Thorneycroft, veteran Conservative Party chairman and another recent critic of Thatcher's policies, resigned and was replaced by Trade Minister Cecil Parkinson, a loyal follower of the Prime Minister's views. Lord Thorneycroft, 72, paved the way for his own departure last month by publicly contradicting government claims that Britain's long recession was over.

Commenting after his dismissal, Sir Ian Gilmour said: "Certainly there is no harm in throwing a man overboard, but it does not do much good if you are steering full speed ahead for the rocks. . .and that is what the government is now doing."

Midterm Cabinet reshuffles are not unusual for British governments and much of the drama of this one was over the fate of Prior, Thatcher's arch-critic inside the Cabinet. As employment secretary Prior had tried to deal in a conciliatory way with Britain's militant trade unions, now demoralized by growing unemployment, and had wanted to do much more to stem the tide of joblessness.

Prior kept his seat in the Cabinet's decision-making Economic Strategy Committee, and said he would make his views known there.

Prior replaces Humphrey Atkins, who had held the tough Northern Ireland post for 2 1/2 years and loyally carried out Thatcher's uncompromising policy of refusing to give in to demands by IRA hunger strikers for political status. Thatcher let it be known that she was pleased with Atkins' "work and awarded him the job of deputy foreign minister, a promotion, and a respite from an exacting and dangerous assignment.

She picked a tough right-winger, Norman Tebbit, to occupy Prior's post as employment secretary.

Prior, who is more independent-minded than Atkins, is expected to favor a more flexible approach to Northern Ireland. Political analysts said it was likely that Prior only accepted the Northern Ireland job after receiving assurances from Thatcher that he would have a freer hand than his predecessors.

The prime minister's moves cleared the way for a more united government economic policy, although one prominent critic, Agricultural Minister Peter Walker, remained. Walker, who is also deeply concerned about Britain's recession, rising unemployment and the degradation of the inner cities, is isolated outside the key economic decision-making circle of Cabinet ministers. Thatcher had been expected to keep him in order to mute his criticisms of her policies.

The leader of the Liberal Party, David Steel, commented that "these changes make the government more hard-line and therefore will be damaging for the country in the short run."