An article yesterday incorrectly identified the political affiliation of International Trade Commission nominee Lyn Schlitt. She is an independent.

President Reagan is about to fill three vacancies on the International Trade Commission, once a sleepy backwater of government but now in the mainstream of government efforts to protect American industries from unfair trade competition, trade sources reported yesterday.

Reagan intends to nominate California lawyer Susan Wittenberg Liebler, who ran into trouble at Senate Finance Committee confirmation hearings last year when she gave incorrect answers to key questions on trade law, and Seeley Lodwick, who lost his job as undersecretary of agriculture for international affairs amid reports that he wasn't aggressive enough in selling wheat to the Soviets, for two of the vacancies, Capitol Hill sources said.

The other nominee is reported to be Lyn M. Schlitt, a lawyer with the Washington firm of Covington & Burling who has appeared before the ITC in trade cases.

Reagan late last year named Liebler and Enrique Leon, a Cuban-born finance and management professor, to fill two of the three ITC vacancies. But both nominees, largely unknown in Washington trade circles, gave incorrect answers at their confirmation hearings on the laws they will administer.

Sources said the president now has decided against resubmitting Leon's name. But he is sticking with Liebler, who also ran into problems involving her political affiliation during the Finance Committee hearings.

Named to a seat that cannot be held by a Republican, Liebler acknowledged under questioning that she had supported Reagan's presidential nomination and worked both on his transition team and as a special assistant to Securities and Exchange Commission Chairman John S. R. Shad. She insisted, however, that she is politically independent.

Lodwick, a Republican, is a former aide to Sen. Roger W. Jepsen (R-Iowa) and former director of a division of the Agriculture Department. Schlitt is a Democrat.