From suburbs to swamps, from drive-in banks to derricks, and from high-rise apartment complexes to low-slung bayou shacks, the third congressional district sprawls across southern Louisiana.

Encompassing all or part of seven parishes (counties), it is more than 100 miles wide, stretching from New Orleans' suburbs to the sugar cane and oil fields of Cajun country, and it houses about 534,000 people.

For seven years, David C. Treen was the district's overwhelmingly popular congressman, but he became governor last month, so the voters there will have to select a successor Saturday.

Four men seek Treen's old job. Becaus of the district's diversity, anyone running for office encounters problems, starting with name recognition. Generally, any candidate from suburban New Orleans, which makes up the easternmost corner of the district and contains nearly 40 percent of its voters is looked on as an alien by the shrimpers and oil workers of Cajun Louisiana. Conversely, candidates from that region are regarded around New Orleans as quaint, lightweight hicks.

James J. Donelon, 35, of the New Orleans suburb of Metairie, who has received Treen's endorsement and features the governor in his commericials, finished a statewide race two months before announcing for Congress. He lost that contest -- for lieutenant govenor -- but his name became widely known.

Because Saturday's congressional election was called barely a month ago, Donelon's opponents have not had time to build that sort of familiarity.

"We've been everywhere we possibly could," said Bonnie Landry, press secretary to state Rep. W. J. Billy Tauzin, 36, of Thibodaux. "He's spend some real long days meetings as many people as possible -- shrimpers, farm groups, port groups, industrial groups, postmen, firemen, and oil and gas groups."

State Sen. Anthony Guarisco Jr., 41, of Morgan City, had done much the same kind of barnstorming, said Jean LaPlace, his press aide.

The fourth candidate -- Robert Y. Namer, 33, of Metairie -- has been less peripatetic, and is given little chance.

Donelon and Tauzin, who are regarded as the front-runners, expect to spend about $200,000 apiece while Guarisco estimates his campaign outlay as slightly more than $100,000.

Donelon became a Republican in February, and he is the only member of that party running in a district where 16,153 of the 259,447 voter are in the GOP. Treen became the state's first Republican congressman since Reconstruction when he was elected from the district in 1972, and he won easy reelections in 1974 and 1976. In 1978, he was unopposed.