Sen. Russell B. Long (D. La.) gave his general blessing to President Carter's new tax-cut package yesterday, but indicated that Carter probably would not get his three biggest proposed "reforms" - an end to two big foreign tax breaks and a crackdown on deductions for martini lunches.

The Senate Finance Committee chairman told reporters he is flatly opposed to limiting deductions for business lunches, as Carter has recommended. "Entertainment is to the selling business the same thing that fertilizer is to the farming business," he said. "It increases the yield."

At the same time, Long indicated he was lukewarm, at best, about Carter's was lukewarm, at best, about Carter's tax breaks. He said he thought if Congress does agred to scrap the provisions, "you ought to give the business community something for it" in return.

The chairman's remarks, his first comments on the Carter tax package since the President proposed it last weekend, appeared to bolster the assessment by other congressional leaders that Carter's three most controversial "reform" proposals probably would not be enacted.

Rep. Al Ullman D-Ore.), chairman of the House Ways and Means Committee, also has expressed doubt about the proposal to trim back business lunch deductions. And Ullman flatly opposes eliminating the two big foreign tax breaks.

Long's comments came as conservatives on the Finance Committee announced plans to renew their attack on a key provision in the 1976 Tax Reform Act that is imposing higher taxes on heirs who sell inherited property. The provision is known formally as the "carry-over basis" rule.

Sen. Harry Byrd (I-Va.) served notice he will try to amend a routine "technical corrections" bill next week to defer the increased taxes "for several years . . . until the committee can understand the full ramifications of it." The provision was one of the few real "reforms" in the 1976 bill.

In discussing the Carter tax package yesterday, Long told reporters he was "generally pleased" with the tax cuts the President had proposed. "We needed a big tax cut, and that's what he recommended," he said. He also agreed the cuts should take effect on October 1.

Although he opposed limiting the deduction for business lunches, Long agreed with Ullman that Congress probably will crack down on other write-offs for business entertainment expenses, such as the cost of manintaining yachts or buying tickets to sporting events.

He declined to predict, however, how many of the $9 billion in "reforms" and other revenue-raising measures the President would get.And be hinted that Congress may expand Carter's proposals to liberalize the investment tax credit for business.

The two foreign tax breaks Carter has proposed to eliminate are the tax-subsidy for exporters under the Domostic International Sales Corp. promoestic International Sales Corp. program, and the ability of multinational firms to escape payment of taxes on foreign-source income they reinvest abroad.