In three weeks, Huntington Bank will take its final big step in its purchase of FirstMerit Corp. The mega-deal, which elevated Huntington to one of the 20 largest banks in the nation, became final last summer. But customers of Akron-based FirstMerit have had to continue using FirstMerit branches, online banking and other services. That changes Tuesday, Feb. 21. That's when 39 FirstMerit and Huntington branches in Northeast Ohio will close, and about 270 FirstMerit branches companywide will convert to Huntington.