On the same day that President Trump announced the United States’ withdrawal from the Paris climate accord, environmentalists took consolation in the closure of three large coal-fired power plants — the kind often blamed as big contributors to climate change. The three plants, two in New Jersey and another in Massachusetts, are the latest in a national trend toward phasing out coal-fired power plants in the face of tighter regulations and competition from cheap natural gas.But in the end, industry officials said they had to close because they were no longer economically viable.“The sustained low prices of natural gas have put economic pressure on these plants for some time.