A subcommittee of the Democratic-controlled House yesterday agreed to a three-month extension of supplemental unemployment benefits, but Senate Republican leaders balked at anything more than a phase-out of benefits for those already on the rolls.

President Reagan said last week he opposes extending of the program, which will expire Sunday, although Republicans in both chambers have indicated that he might agree to a phase-out.

As drafted by the Ways and Means subcommittee on unemployment compensation, the House measure would continue benefits through June 30 for unemployed workers who have exhausted their regular 26 weeks of jobless payments.

Those on the rolls would continue to receive eight to 14 weeks of benefits, depending on unemployment levels in their states, while those who newly qualify during the three-month period to June 30 would get four to eight weeks.

As approved by the subcommittee, the measure has an estimated cost of $270 million, as opposed to roughly $100 million for a phase-out limited to those currently on the rolls.

The subcommittee also approved legislation providing for permanent revision of jobless benefits for the long-term unemployed, but it was written as a separate bill and is expected to be shelved for further study when the Ways and Means Committee considers the issue Tuesday.

The temporary extension, sharply scaled back from earlier versions that were circulated, was modified largely to achieve a compromise that Reagan would sign.

"It's a reflection of our recognition of reality that he'll veto it unless we give him a rationale to change his mind," said House Majority Leader James C. Wright Jr. (D-Tex.). "That's what we want, a bill signed, not a Pyrrhic victory," Wright added.

But it was not clear whether Reagan would approve the legislation if it includes an extension for newly qualified beneficiaries.

Senate Finance Chairman Bob Packwood (R-Ore.) said his committee will consider unemployment legislation Tuesday, but added, "I'm not going to try for anything more than a phase-out or it will be a sure veto."

Moreover, Rep. Carroll A. Campbell Jr. (R-S.C.), who cast the subcommittee's sole dissenting vote against the extension, warned that it could run into procedural obstacles from Republicans as well as a veto threat unless it is trimmed to a simple phase-out.

For instance, if Republicans insist on following usual procedures for consideration of legislation, the bill might have to be brought up under rules requiring a two-thirds majority for passage or be delayed until after Congress returns from its Easter recess April 15.