A proposed 13 percent rate increase for water and sewer users in the Maryland suburbs failed to win approval yesterday when the three Prince George's County members of the Washington Suburban Sanitary Commission voted to block it.

The votes of the Prince George's members -- who said they wanted to delay the decision, in accord with the wishes of their county executive and County Council -- effectively postponed until January a final decision on the rate increase.

Commission chairman David R. Scotton said yesterday that he and the other two Montgomery County commissioners voted for the rate increase "even though my county government requests we defer the increase . . . We must do what is financially responsible."

Robert S. McGarry, the general manager of the bicounty agency, and warned that without a rate increase, the agency would be running a deficit of more than $2.3 million by next June, which he said would "badly jeopardize" the agency's bond rating.

If no rate increase is voted in January, agency officials said, an increase of "well over" 31.5 percent will be needed when the 1981 fiscal year begins July 1.

Agency officials attribute the agency's recent financial difficulties to a combination of factors, including last spring's decision to decrease rates by 10 percent and a summer of heavy rains that cut down on water use by WSSC customers.