William Donald Schaefer won a third term as mayor of Baltimore today, easily turning aside the challenge of Republican Samuel A. Culotta and leading Democratic sweep of top city offices.

Also winning, although by a much smaller margin, was a rent-control plan that had become the election's hottest issue. With all ballots counted, the plan had about 52 percent of the vote.

The measure will set up a landlord-tenant board to establish rent levels and rule on proposed rent increases. The panel will also decide whether landlords could demolish rental properties or change their use.

Renters make up about 70 percent of the city's population.

Schaefer's lead over Culotta was almost 5 to 1, Culotta, a lawyer was an aide to Gov. Theodore McKeldin in the 1950s and is a former member of the state legislature.

Culotta's campaign centered on attacks of Schaefer, although he also proposed providing better city services by hiring 400 more police officers and making available more low-income housing.

Culotta's most serious charge was that Schaefer failed to detect bid rigging in the awarding of city contracts from 1971 to 1975, which resulted in the conviction of a city official, by "leaving the room" at appropriate times during Board of Estimates meetings.

Schaefer responded to few of Culotta's attacks, spending his time running city government and holding news conferences to announce new federal grants to the city.

Democratic incumbents easily won the races for City Council president and comptroller, and Democrats won all 18 City Council seats. Democrats hold a 9-to-1 registration edge on city voter rolls.

Walter S. Orlinsky defeated Republican Richard L. Andrews in the council president's race while Hyman A. Pressman defeated Republican St. George I. B. Crosse III in the comptroller's contest.

Voter turnout was reported lighter than the 36 percent predicted by city elections officials. In September's primary, just 28.5 percent of the city's 380,062 registered voters cast ballots.