An energy company planning to build an 800-megawatt wind farm off the coast of Martha's Vineyard has cleared a key milestone now that it has reached deals to sell the power to three major Massachusetts utilities. The state Department of Energy Resources said Wednesday that the Vineyard Wind joint venture signed contracts to sell the power at an average cost of 6.5 cents/kilowatt hour, in 2017 dollars, and would result in slightly lower bills for customers. The agency said this would be the largest procurement of offshore wind power in the United States. Vineyard Wind was picked by the three utilities — Eversource, National Grid and Unitil — in May to build the $2 billion project. It expects to begin construction next year, and bring the wind farm online in 2021, although the contracts still need to be approved by the state Department of Public Utilities. The venture is also still lining up state and federal permits.