Lobbyists for the nation's private charities renewed their campaign yesterday for a $3 billion tax break for small contributors that they argue is needed to spur charitable donations in the United States.

Backed by congressional sponsors of the measure, representatives of more than two dozen charities appeared before a Senate Finance subcommittee to plead that charitable giving is suffering and needs new incentive.

The campaign is a revival of a 1978 lobbying effort that saw the legislation approved tentatively by the House Ways and Means Committee but later overturned over the measure's high cost.

The charities are upset because a 1977 law repealed the charitable deduction for taxpayers who do not itemize as part of a move to simplify tax-return preparation by replacing some deductions with a larger standard deduction.

Although taxpayers who itemize still may claim a separate deduction for the donations, the charities complain that smaller contributors who usually file the short form, no longer have as much incentive to give.

Both Carter administration officials and many private economists challenge that position, asserting that small contributors, such as those who donate to churches or local civic clubs, tend to give the same with or without a tax break.

Yesterday, Donald C. Lubick, assistant secretary of the Treasury for tax policy, told the subcommittee that the charities' bill would only provide a "windfall for existing contributors, and would not spur additional giving.

Lubick said the measure also would add seriously to the complexity of filling out the short form and would present extra administrative problems for the Internal Revenue Service.

The renewed campaign to push through the charities tax-break legislation was expected to be one of several such moves by special-interest groups to have their proposals included in any tax-cut bill this year.

Although both President Carter and key congressional leaders have eschewed any tax cut as inflationary, backers of several narrower tax-break proposals are pushing to have them considered early in case Congress changes its mind.

Similar efforts are underway by proponents of faster depreciation writeoffs for business and larger deductions for Americans living overseas. Carter has said he will consider proposing A tax cut if the recession proves deep.