Somerville residents will have a chance to weigh in on plans for new MBTA stations in Gilman Square, Magoun Square, and Ball Square next week. City planners will be on hand to take feedback in open studio sessions from 11 a.m. to 2 p.m. on Wednesday and next Thursday at the Armory, 191 Highland Ave. A public meeting will also be held from 6:30 to 8:30 p.m. Wednesday at the Armory, where residents can contribute feedback. Renderings, maps, and graphics related to the project will be exhibited at the Armory from 4:30 to 9:30 p.m. Thursday. The planned stations are part of the Green Line extension project, which will extend the subway line from Lechmere station in Cambridge through Somerville to a terminal station in Medford. It was most recently projected to be completed by 2020 and cost $1.4 billion.