Organizers of Poland's new independent trade union movement said today that 3 million workers from about 3,500 factories and institutions want to join the new unions, giving the movement the same number of people as belong to the Polish Communist Party.

The figure, the first informed estimate of the size of the rapidly expanded 2 1/2-week-old movement was given in a communique following a national meeting here yesterday of representatives from Poland's new union groups.

The existence of free trade unions, a key concession won by strikers along the Baltic seacoast last month, challenges the authority Polish officials once had over workers through the old official trade unions. Such a large estimate of the strength of the worker's new movement only adds to the perception of it as a power group emerging parallel with the Country's Communist Party.

Representatives of the free unions are seeking to clarify what is still a murky picture of what the structure of their young movement will be. In an important move yesterday, they created a national commission comprised of delegates from the founding committees of the new independent regional unions that have sprung up in at least 33 areas.

The commission, which will be headquartered in Gdansk -- a center of militant labor activity during the summer strikes -- is expected to be a first step toward a national federation of independent unions.

It plans to register formally in a Warsaw court early next week for nearly all the new union groups, thus making it seem legally as though there will be just one grand, new independent union in Poland. But the exact role the commission will play in the movement remains unsettled.

Some want it to have simply a coordinating function, others are pressing for it to have firm controlling powers.

The communique issued today by representatives who attended yesterday's meeting listed the following as the immediate tasks of the commission:

Organizing the joint activities of new unions for the whole of the country.

Assuring the new unions will take part in the formulation of the new trade union law.

Guaranteeing the new unions will take part in drawing up social political policy.

Seeing to it that the new unions will control the fulfillment of the agreements between the workers and the government.

The commission will also have to define the form of individual union activities within the framework of one single union, the communique said.

In another development, the Polish press agency disclosed more detailed information about the budget cuts for 1980 and 1981 made necessary by the promise of higher wages and social benefits included in the strike agreements.

Subsidies for radio and television, for the cinema industry and for sports will be trimmed because they do not represent pressing national needs or because they sometimes involve excessive expenditures, the press agency said.

Costly research work will be shelved, including Poland's arctic program.

"The axe will also fall on those expenditures which evoke a great deal of public criticism," the report said, especially those for representation, including not only reception parties, but also various events arranged by individual industries.

Further, the govenment plans to eliminate "excessive incomes" paid to some working groups and to cut the investment programs of state companies.

It also has decided to lay off an unspecified number of government managers, particularly in the industrial administration division, "which can be trimmed without the slightest harm to the national economy," the press report said.

On top of all this, Warsaw officials were said to be considering reductions in spending on highway construction and a reconstructiion of urban transport systems.

To raise more money, the government expects to offer "more attractive savings schemes" for Poles and to institute an advance payment plan to encourage them to save up for "various attractive goods in short supply on the market," the report said.

All told, these measures were said to offer Poland savings of $400 million this year and between $730 million and $1.1 billion in 1981.

"The aim is to acquire the resources necessary for the sweeping social programs which have been announced by the government," the news agency said, programs which Warsaw officials say will cost about $3.7 billion altogether.

When Poland can in fact afford its costly settlement with workers depends largely on how successfully its recovery effort goes. So far, the country still is not completely back to work.

Polish television today reported strikes under way in 16 enterprises, and in some cases, plants were said to be striking for the second or third time. The TV announcer repeated government warnings that "every day of loss" caused by presistent strikes "hampers fulfillment of agreement reached between government and workers."

Even in factories not on strike, worker discussions about special grievances and the organizing of new unions are said to be disrupting the pace of productions.