Three conservative Republican senators were nominated yesterday for the Senate Judiciary Committee as part of an apparent plan to prevent Sen. Charles McC. Mathias Jr., Maryland's liberal Republican, from becoming ranking minority member of the important committee.

The GOP's Committee on Committees voted to put Sens. Paul Laxalt (R-Nev.), Orrin G. Hatch (R-Utah) and Malcolm Wallop (R-Wyo.) on the Judicary Committee with the three Republican holdover members, Sens. Strom Thurmond (R-S.C.), William L. Scott (R-Va.) and Mathias, the only nonconservative in the group.

While he has yet to make a public announcement, there is every indication that Thurmond, who once ran for President as a Dieiecrat, will use his own seniority to block Mathias, who has progressive civil rights record, from becoming the Judiciary Committee's ranking minority member and spokesman for its Republicans.

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The nomination of the three conservative Republican senators from the West to Judiciary appears to assure that Thurmond would have support from them to defeat a challenge by Mathias. Mathias declined yesterday to comment on the situation.

In order to become ranking minority member of the Senate Judiciary Committee, Thurmond would have to give up being ranking member on Senate Armed Services, enabling Sen. John Tower (R-Tex.), another conservative, to succeed imas ranking member there. Tower, however, then would have to make way on the Senate's Banking, Housing and Urban Affairs Committee, allowing Sen. Edward W. Brooke (R-Mass.) to replace him as ranking member there.

Sen. Paul Sarbanes (D-Md.) was the only freshman senator nominated yesterday by the Democrsatic Steering Committee to the Prestigious Senate Foreign Relations Committee, Sarbanes' first choice.

Maryland's junior senator also was nominated for the Banking, Housing and Urban Affairs Committee, his third choice but one valued by Sarbanes as putting him in a good position to work on housing, rapid-transit and consumer needs.

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The committee assignments recommended yesterday by the Senate Democratic Steering Committee and the Republican Committee on Committees will be acted on today by separate party caucuses.

Along with Sarbanes, two other Democratic senators, John Glenn (Ohio) and Richard Stone (Fla.) were added to the Senate Foreign Relations Committee, but not without a challeng to Stone from Sen. James Abourezk (S.D.)

Abourezk said later that moments before the vote on Stone's bid to move to Foreign Relations he told the closed session of the Democratic Steering Committee that Stone "is very, very pro-Isaeli. In think I ought to be on there to give the committee some balance." Abourezk lost, 14 to 10.

After a lunch break, Abourezk did not seek the extra Foreign Relations seat added for Sarbanes, it having been pointed out that there is a rule against two senators of the same state and political party serving on the same committee. Sen. George McGovern (D-S.D.) already is on Foreign Relations.

The Democratic Steering Committee voted to put conservative Sen. James B. Allen (Ala.) on the Senate Judiciary Committee, only to be told by Allen that he didn't want to serve on the committee, which he tried unsuccessfully to join two years ago, losing his bid by a 9-to-9 vote.

Allen's friends on the Steering Committee moved yesterday to put him on Judiciary after Sen. Thomas F. Eagleton, chairman of the now defunct Senate District Committee, was allowed to move to the Government Affairs Committee that will include District affairs while regaining his previous seniority, which puts Eagaleon senior to Allen.

Virginia's two senators are scheduled to remain on their present committees. Sen. Harry F. Byrd Jr. is on Finance and Armed Services and Scott is on Judiciary and Armed Services.