Opening the way for a possible compromise on tax cuts, one of the most influential Democratic opponents of the Reagan administration's three-year tax cut plan offered yesterday to discuss his differences with congressional Republicans and the White House.

Rep. Dan Rostenkowski of Illinois, chairman of the House Ways and Means Committee, said he still believes a 10-percent-a-year, three-year tax cut is too much and he favors a tax plan with "flexibility" because of "the unpredictability of this economy."

"I'd like to discuss my theory with the ranking [Republican] member of my committee, Barber Conable, [and] with the president if necessary," Rostenkowski said on the CBS program "Face the Nation."

On NBC's "Meet the Press," Treasury Secretary Donald T. Regan said the administration is willing to consider a smaller tax cut if it would satisfy the president's goals of increasing saving, investment and productivity.

"Can they accomplish the same thing?" Regan asked. "If they could show how to do it, we'd like to listen."

The conciliatory statements came after the Democratic-controlled House overwhelmingly endorsed President Reagan's budget plan last Thursday and rejected an alternative proposed by the Democratic leadership.

Max L. Friedersdorf, the administration's congressional liaison opened the doors to a White House compromise when he suggested in a weekend interview:

"If I were Danny Rostenkowski, I would be coming in to talk with the president about his program and get the best deal I can before the tidal wave rolls over him."

Rostenkowski clearly was listening. In his appearance yesterday, he repeatedly stressed his willingness to work with the administration while emphasizing the Democrats' view of how much and when taxes ought to be cut.

Democrats on the House tax-writing committee "continue to feel they can't vote for a three-year program," he said. "I'm convinced most of these members are going to be for a smaller tax cut."

Expressing fears that tax cuts might fuel inflation despite administration assurances to the contrary, Rostenkowski said, "We ought to be talking more about what we should be doing with the unpredictability of this economy."

"I'd just feel more comfortable knowing that the president has a full observation of what I see as problems in the economy in the next three years," he said.

"I'm trying to work with the president within the parameters he has outlined . . . I certainly don't think my door is closed."

Meanwhile, Regan was sending Democrats the message that "what the administration is doing right now is we're standing firm."

"We have our own proposals," Regan said. "We've seen none that are better to get the country moving the way we think it should be moving, to get more savings, more investment and to give people an incentive to do better.

"If there are other proposals out there that would farther this program or do better than this program, we're willing to listen to whomever has it. People know my phone number," Regan said.

Rostenkowski and Regan also increased the chance for compromise on Social Security issues. Rostenkowski said that some cuts in benefits will be made as part of this year's budget but that the major issue is "the solvency of the program."

"I believe this has to be a bipartisan effort," he said, urging Congress and the White House to "sit down and diagram where we think we are going and what we think we're going to do." The resulting compromise, he added, "ought to go through the House and Senate without any amendment."

Regan suggested that the administration may want to go further than the Senate did in limiting cost-of-living increases in Social Security benefits and might move to restrict some benefits.

He cited the example of a federal employe who retires on a government pension, then works in private business and qualifies for a second retirement plan under Social Security.