Arizona, California and Nevada on Monday proposed a plan to significantly reduce their water use from the drought-stricken Colorado River over the next three years, a potential breakthrough in a year-long stalemate over how to deal with a rising problem that pitted Western states against one another. The plan would conserve an additional 3 million acre-feet of water from the 1,450-mile river that provides water to 40 million people in seven U.S. states, parts of Mexico and more than two dozen Native American tribes.