The Defense Department's research director yesterday launched the idea of doubling the size of the mobile Midgetman missile so it could carry three warheads instead of one, a proposal that seemed to blow up on the pad of a House subcommittee.

"We could save America $20 billion," Undersecretary of Defense Donald A. Hicks told the Armed Services subcommittee on research and development. He said 500 Midgetman single-warhead missiles are estimated to cost up to $50 billion, while a force of 170 launchers, each with three warheads, could deliver the same load for $20 billion less.

Hicks said the Defense Department is considering the triple-warhead option but has not reached a decision.

The new Midgetman would weigh 75,000 pounds compared with the 33,000-pound version under development, Hicks said, but still could "dash" across the desert West or along roads to escape incoming Soviet nuclear warheads. Hicks said he saw no technical obstacles to saving money through construction of a larger Midgetman.

"I don't think we ought to go loading up Midgetman," said Rep. Dave McCurdy (Okla.), second-ranking Democrat on the subcommittee, after hearing Hicks testify that the proposed three-warhead missile and its protective covering would weigh a total of about 250,000 pounds.

Anthony R. Battista, the subcommittee's staff director, termed the three-warhead Midgetman "MX Junior or at least a Minuteman 3." MX is the blockbuster land-based missile that has run into strong congressional opposition while Minuteman is an older missile deployed in silos.

"You wouldn't get as much micromanagement [from Congress] if we had a more consistent policy coming out of your building," Battista said in complaining about the twists and turns of Reagan administration strategic policy. "Where are you going?"

The administration with the help of a number of congressional leaders put together a shaky coalition of MX supporters on the condition that the single-warhead mobile Midgetman was developed concurrently as recommended by a 1983 presidential commission headed by retired Air Force lieutenant general Brent Scowcroft.

Putting the 10-warhead MX in existing Minuteman silos would just draw more Soviet fire, critics said, while spreading mobile Midgetman missiles over a wide area would add to stability by making a knockout surprise blow look impossible to Soviet gunners because they could not destroy all retaliatory warheads.

Rep. William L. Dickinson (Ala.), ranking Republican on the Armed Services Committee who helped put together the MX-Midgetman coalition, said after hearing Hicks testify, "If he is an opponent of Midgetman and wants to substitute for it, he shot the [president's strategic] program right in the head."

Sen. Albert Gore Jr. (D-Tenn.), another forger of the coalition, said of the idea of switching to a three-warhead Midgetman: "It's difficult to image a program that would do more damage to Scowcroft consensus or the ICBM [intercontinental ballistic missile] modernization program."