Three wind turbines that have been idle during the past eight months while towering over the Providence waterfront will get their first test spins this week. The 364-foot-tall turbines will be tested on Tuesday and probably put into full operation soon afterward, a spokeswoman for the Narragansett Bay Commission told the Providence Journal. They are the only large turbines in Providence and are among the largest in the state, reaching the same heights as one in Portsmouth and another being constructed in North Kingstown. The 1.5-megawatt turbines are expected to provide 40 percent of the power at the commission’s Fields Point waste water treatment plant, where the electricity bill totals $2.5 million a year. The turbines, which are visible from Interstate 95, were up in a matter of weeks, but it took much longer to connect them to the treatment plant and the regional power grid.