The owner of two of New England’s most polluting power plants yesterday announced a three-year, $17 million plan to reduce emissions that cause smog and acid rain. U.S. Generating Co., which acquired the plants last year, said it would cut nitrogen oxide (NOx) emissions by 23 percent and work toward reductions in sulfur-dioxide (SO2) emissions. Environmentalists have been pressuring the company to close or clean up the plants, which are located in Salem and Somerset, Mass, and they remained unhappy that the power company wasn’t making greater strides toward switching to lesser polluting fuels such as natural gas.