Republicans on the Senate Armed Services Committee, seeking to fend off a defense-spending freeze for next year, told the Senate GOP leadership yesterday that they agree with a cut of one-third, but no more, of President Reagan's proposed military buildup for fiscal 1986.

Headed by committee Chairman Barry Goldwater (R-Ariz.) and including the panel's 10 Republicans, the group said in a letter to Senate Majority Leader Robert J. Dole (R-Kan.) that it continues to support Reagan's request for a 5.9 percent Pentagon spending increase next year after accounting for inflation.

As part of a deficit-reduction package, however, the Republicans indicated that they could accept an after-inflation increase of 4 percent but added that "in no case" could they accept a lower figure.

Republican leaders seek a greater cut in the defense buildup, possibly even a freeze at current budget authorization levels, and it was unclear yesterday how Armed Services Committee Republicans' position would affect compromise efforts.

One source said any sign of flexibility by the strongly pro-defense committee would help achieve a GOP consensus, blocked for the last couple of weeks largely because of resistance to defense cuts by the Pentagon, the White House and the Armed Services Committee.

Obstacles continued to appear in the path of the Republican leadership's effort to arrive at a plan for halving deficits to less than $100 billion by 1988.

Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) is expected to tell a meeting of Republican senators today that the spending cuts required to meet the goal appear even larger than expected earlier in the year.

Because of revenue estimate revisions and other changes, next year's target for cuts is $64 billion, $10 billion more than projected earlier, sources said yesterday. By 1988, required cuts rise from $118 billion to $151 billion, they said.

Also, sources said the Congressional Budget Office is expected to release a report later this week calculating that Reagan's budget would produce a $171 billion deficit by 1988, rather than the $145 billion deficit the administration projects. Reasons reportedly include different projections for interest rates and economic growth over the three-year period.

Domenici also is expected to outline a plan to reach the target by imposing a modified one-year spending freeze coupled with virtually all program cuts proposed by Reagan and other options outlined earlier by the Budget Committee.

Under the modified freeze, all programs would be kept at current spending levels except defense and programs targeted at the poor, both of which would be expanded only to cover costs of inflation.