They played for gangster Al Capone and for Harlan County, Ky., coal miners who kept time with their pistols. They traveled the herb medicine show circuit and slept by the side of the road. Once in 1934 they emerged from a Chicago theater after playing between movies and found John Dillinger sprawled on the sidewalk, surrounded by FBI agents.

They've been through a lot together, these three who call themselves "Martin, Bogan and Armstrong" and one senses their 46-year bond in their style of music -almost as if they're swinging with one instrument - and an obvious affection and bemused tolerance for each other's idiosyncracies.

Mandolin player Carl Martin, 71, with his gimpy leg, guitarist Ted Bogan, 67, with his impish smile, violinist Howard Armstrong, 63, with his beret or leather headband - how well these men know each other. They address each other as "Mr.," and they laugh as they share the spotlight on "Lady Be Good," "Corinna, Corinna," "Sweet Georgia Brown" and many originals.

They don't talk much about the experiences that have helped shape their music - jazzn swing, spirituals, popular and ethnic numbers - but a listener can sense them between the boats. Here are three men who have probably seen about everything, and their music - jazz, swing, spirituals, of swing also keeps them young, as enchanted audiences last weekend found during their appearance at the Childe Harold.

The three first came together with another member as the four Keys String Band in 1931. They were then, and remain today, a rarity, a black string band. They traveled widely to find their audience, gearing everything towards payday - when people could afford entertainment.

In June 1937, when Joe Louis became the first black heavyweight champion of the world, Martin, Bogan and Armstrong were trying to make a living in Chicago's multi-layered ethnic neighborhoods. One night they stumbled into a restaurant and focused through a cigar haze upon a group of hostile Sicilians.

"You think Joe Louis is a nice man?" shouted a voice.

"I don't know a thing about Joe Louis." Armstrong recalls saying as he backed to the door. He backed right into Martin who backed into another musician who backed into a bass player who blocked the door. "Well," thought Armstrong, "all is lost."

Instinctively he started talking in the snatches of Italian he had picked up in his travels, insisting, "We can play Italian music!" The Sicilians relaxed. "I said 'Everything's over, boys, get you axes out," Armstrong chuckles. And they took out their instruments.

Martin, Bogan and Armstrong wound up playing that restaurant for more than a year at a time when most musicians were out of work and starving. They played anywhere and everywhere - square dances, bar mitzvahs, funerals, weddings, picnics, churches, on record and on the radio. They came to know hundreds of songs, in almost a dozen languages.

They played to please their audience and sometimes that audiences could be tough. But regular employer Al Capone the musicians remember as a kind patron. "That was a little more steak on the table," says Martin. "We were happy to be able to play for anybody with a little money."

It was an exciting an vital time and it ended in the '40s with World War II when families and other jobs took over. After the war, the musicians tried to return to their former places of employment but the "syndicate" had, as Martin rememberes, put juke boxes in all the clubs. It made for more efficient control of the clubs, but it was also "kind of rough. A lot of prefessional men had to put their violins up on the shelf, 'cause you can't buck the syndicate."

The three remained friends and when all of them had retired from their non-musical professions by 1970, they were enticed into forming again - after a 30-year break - for several folk festivals, including the Festival of American Folk Life in Washington, where they were a tremendous success. In 1975, they were part of a State Department goodwill tour of South and Central America which was high-lighted by the wife of Nicaragua's president requesting "some more low-down, dirty blues."

Blues is but one aspect of the populist appeal of the trio's music. The fact that they draw from everywhere may help explain their lasting and cross-generational appeal.

"We have a type of understanding and rapport that you can't eradicate," says Armstrong. "You're never dead till the undertaker brings the hearse up toe your door. This is something that we do spontaneously."