In an effort to make the MX land-basing system acceptable to the Reagan administration, Pentagon supporters have again modified the controversial sheltering plan by eliminating some costly arms control features demanded by the Carter White House, according to sources in the Defense Department and on Capitol Hill.

According to Pentagon officials, changes in the Carter MX plan would lower cost of the system by $3 billion, and also would cut by as much as 15 percent the land needed in Utah and Nevada for the multiple-shelter plan.

The new plan for the $50 billion intercontinental ballistic missile is one of many options being presented to a special MX review committee established by Defense Secretary Caspar W. Weinberger and chaired by Nobel Prize winner Dr. Charles Townes.

"This is the option we believe they will end up accepting," a general involved in the MX program said.

The modified plan is but one part of a package of old and new strategic weapons programs being reviewed by Reagan defense officials as they draw up their strategy to meet what they have called the decisive Soviet lead in ICBMs. The Carter MX basing plan has drawn criticism from both Weinberger and President Reagan.

Other elements being pushed by the Pentagon MX group include accelerated building of an antiballistic missile system to defend the land-based MX and rapid development of a smaller, single-warhead ICBM that could be produced and deployed by the thousands if the Soviets keep up the pace of their ICBM program.

The Weinberger-appointed committee is being briefed on various MX basing plans and is to report to the secretary in June. Pentagon and Captiol Hill sources said that although the group is in early stages of review, several members have already spoken favorably of the new, modified MX plan and the associated mix of strategic missile programs.

Under the Carter plan, each MX missile was to travel in a transport among 23 shelters -- a deployment designed to hide its location but still assure the Soviets that only one missile was hidden in each grouping of shelters. In another gesture toward arms control, the shelters were to be horizontal, like garages, with ports in the top that could be opened to show that only one missile was in each 23-shelter grouping.

The modified plan would drop the separate 23-shelter-per-missile sections and "pack" valleys in Utah and Nevada with as many combinations of missiles and shelters as they could take.

Most valleys could accomodate four MXs and their 92 shelters but, according to one Pentagon official, one large valley is being allocated 10 missiles and 230 shelters.

Thus the 200 MX missiles could travel among and be hidden in any of the 4,600 shelters. This would allow the Air Force to cut the number of multimillion-dollar missile transporters from 200 to perhaps 50, according to one source. It would also reduce the need for separate maintenance facilities from 200 to far fewer.

As a result of the packing idea, the Soviets would have to count the MX missiles as they are constructed, rather than after they are deployed, if they wanted to be certain how many the United States really has.

This is because the new plan eliminates the portholes on top of each shelter -- holes that were to be opened on a regular schedule to prove to Soviet photo intelligence satellites that there was only one missile in each of the separate 200 groupings.

Packing the valleys also would reduce the number of roads needed, sources said.

Because Reagan strategists believe that Soviets will have enough warheads to defeat the MX and its 4,600 shelters by the late 1980s, when the full system would be deployed, the new plan proposes pressing ahead with advanced development and initial production of a mobile ABM system that could be used to protect at least 100 of the 200 MX missiles.

Such an ABM system, proponents argue, would make it possible for the Soviets to be sure of destroying an MX missile even if they fired two warheads at each of the 4,600 shelters. However, a decision to deploy such a system, these sources said, would require the United States to renounce, or at least amend, the 1972 ABM treaty. That agreement now limits both sides to only one active ABM site, and each site can have only 100 missile interceptors.

Another Pentagon plan, to deploy an ABM system around the radar site in North Dakota, reported Friday in The Washington Post, would not violate the 1972 treaty.

As a final hedge against a future Soviet warhead inventory increase -- from the 10,000-12,000 now to 20,000, which some intelligence experts fear -- the new plan calls for development of a small, single-warhead ICBM.

This new missile would be one-third the size of the Minuteman ICBM, but its warhead would be powerful and accurate enough to destroy a giant Soviet ICBM in its silo. Up to 4,000 of the small missiles would be produced.

Most would be deployed in hardened silos on military installations around the country. The remainder would be linked to mobile launchers, either on trucks that would move inside military reservations during times of crisis, or airplanes capable of taking off during a crisis and air-launching the small ICBMs.