The wife of Adm. William H. Duvall was explaining to a group of guests at a Georgetown party last night what life had been like among her three talented sons. "Bodge would play the guitar and sing country and western songs, Bub would sing his opera and Jack would do his tenor."

Bodge, the 48-year-old middle son, then declared to the crowd, "And then my mother would mouth all the words." Just to keep to one's bearings, it should be noted that Bodge better known outside his family as Robert Duvall, who won an oscar for his 1972 portrayal of lawyer Tom Hagen in "The Godfather." He is now drawing virtually unanimous praise for his performance as the mad, surfboard nut Lt. Col. Kilgore in "Apocalypse Now," Francis Ford Coppola's $30-million epic about the Vietnam war.

Robert Duvall is better able than most major stars of the screen and stage to keep a low profile in order to avoid upstaging others present, like his older brother, baritone William (Bub), who was the guest of honor (and a principal performer) at a musicale and buffet given last night at the home of the MacKenzie Gordons.

Robert Duvall is an unusual kind of star. He is neither a matinee idol nor a "character actor," being more commanding than the latter but less heroic than the former. He is not a candidate to be another Redford, but he is hardly at any loss for meaty parts. His burgeoning career, which began in the 1960s with "To Kill a Mockingbird," has not been particularly affected by considerations of glamor.

The party originated the previous week when the elder Duvalls had informed the Gordons that all three sons would be in town, and they had agreed that it might be a good time for Bub to display some of the talents he has been showing at the University of Wisconsin where he teaches. For 45 minutes he, soprano Patricia Nelson and pianist Jeffrey Hollander gave plentiful amounts of song, ranging from passionate Italian verismo opera to the lyric pathos of Rodgers' and Hammerstein's "This Nearly Was Mine," transposed up from the bass line in which Ezio Pinza introduced it in "South Pacific."

Robert Duvall, dressed in a white suit with vest and Edwardian zippered leather boots, did not speak up until it was suggested that Hollander improvise "Tea for Two," and then he shouted out his preference for a "jazz" version. Later he described his recent experiences in London with one of the most famous performers to merge jazz techniques with those of classical music, French violinist Stephan Grappelli. "He had this young guitar player with him named John Ethridge who was just unbelievable," said Robert Duvall.

"He reminded me of the intensity of the gypsies, and that's what the movie I'm now making is all about," said Duvall. "But in jazz that's not unusual. After all, Reinhardt (the famous guitar player) was a gypsy. They are crazy people, a sort of renegade mafia. But they are also wonderful."

Robert's only musical contribution in recent months was the playing of Wagner operas on speakers from his battle helicopters in "Apocalypse Now." "I understand," he said, "that some of the officers actually used music for psychological purposes that way. That was the renegade branch of the Army."