Maryland Gov. Harry Hughes, hoping to boost his U.S. Senate campaign in the home territory of one of his chief rivals, pledged $20 million in state funds yesterday for three major highway construction projects in the congested northern and eastern reaches of Montgomery County.

Hughes announced the election-year plum to an enthusiastic audience of Montgomery legislators, government officials and about 100 local real estate agents in Rockville minutes after picking up the first formal group endorsement of his uphill Senate campaign.

The Hispano-American Chamber of Commerce of Maryland Inc., which represents about 150 small businesses in the Washington suburbs, endorsed Hughes' candidacy in gratitude for the governor's efforts to save a troubled shopping center in Prince George's County that is geared to Hispanic consumers, a spokesman for the group said.

Hughes, who has gone to some lengths in the past 13 months to bestow state government money on populous Montgomery and Prince George's counties, said the $20 million pledge would make Montgomery the single largest recipient of public transportation funds in the state.

The prosperous county already plans to spend $250 million of its own money during the next six years on highways, more than a third of which are state roads. During the same period, the Maryland government plans to spend more than $780 million on highways and the Metro subway and bus systems in Montgomery, Hughes said.

The three projects that will receive special funding are:

*Widening to six lanes two sections of Rte. 29 in eastern Montgomery, between New Hampshire Avenue and Industrial Parkway, and between Greencastle Road and Rte. 198. This project, to start in the spring of 1988, will cost $8 million.

*Adding two new lanes to the Mid-County Highway, which is under construction in the booming Gaithersburg area between Montgomery Village and Shady Grove Road. This $7 million project is scheduled to start in the fall of 1987.

*Widening to four lanes Dr. Bird Road (Rte. 182) in the Olney area. Construction on the $5 million project is scheduled to start in 1989.

Maryland Transportation Secretary William K. Hellmann, who attended yesterday's announcement, said the $20 million for the projects would come from gasoline tax and automobile title fee revenue, which has risen recently as gasoline prices have fallen and car sales have climbed.

Hughes, who trails Rep. Michael D. Barnes of Montgomery County and Rep. Barbara A. Mikulski of Baltimore in the race for the Democratic nomination to the Senate, acknowledged yesterday that the new highway money might help him in the district where Barnes enjoys enormous popularity.

State Sen. Laurence Levitan, who helped negotiate with state officials to obtain the road money, persuaded officials to hold the event before a prime political audience: the annual gathering of Montgomery lawmakers and local real estate agents at a Rockville hotel.