Polly Shackleton, whose D.C. City Council seat in Ward 3 was saved recently when the council redrew Washington's ward boundaries, has decided to seek reelection for a third term.

Council member Hilda M. Mason has decided to seek reelection for a second term to her at-large council seat, and Ward 6 council member Nadine P. Winter says she will run for a third term.

Seven of the council's 13 members are up for election this year--six Democrats and Mason, a Statehood Party member. Council members serve four-year terms.

"I have invested a lot of my life to get an elected government in the District even with limited powers," said Shackleton, who has served on the council 14 years, longer than anyone else. "I just feel that I am responding to what people are saying, that it is important for me to continue doing this."

Shackleton had her seat saved in November even though the council voted to move most of Georgetown out of Ward 3 into Ward 2 as part of its redistricting plan to balance each ward's population.

But a two-block-wide sliver of Georgetown containing Shackleton's home was kept in Ward 3.

Mason, who was elected to the council four years ago, said she wants to retain her seat "so I can help Floretta McKenzie," the new D.C. superintendent of schools. Mason served on the school board before she came to the council.

Ruth Dixon, a former president of the D.C. League of Women Voters, and Mark L. Plotkin, a political consultant, announced they will oppose Shackleton in the Sept. 14 Democratic primary.

Johnny Barnes, who ran unsuccessfully for a City Council seat from Ward 7 in 1980, is running for one of the council's two at-large seats that are up for election this year.

Other City Council members whose terms expire this year are chairman Arrington Dixon, who is trying to decide whether to run for mayor or his present position; at-large council member Betty Ann Kane, who has already announced that she is running for mayor and therefore will give up her council job; David A. Clarke of Ward 1, who has already filed to retain his seat; and William Spaulding of Ward 5, who could not be reached for comment. Former council member Douglas Moore has said he will seek Spaulding's seat.