The three Democrats running for the state House of Representatives’ 24th Middlesex District seat will square off in a forum Thursday from 7 to 9 p.m. at the Senior Center, 27 Maple St. The Arlington Democratic Town Committee is sponsoring the candidates forum in preparation for the Sept. 6 primary election. The 24th Middlesex has been redistricted, and will represent precincts 2, 4, 8, 10 and 12 in Arlington, all of Belmont, and two precincts in Cambridge’s Ward 11. There is no incumbent in the race; the seat has been empty since January, when the incumbent, Will Brownsberger, was elected to the state Senate. The Democrats vying in the party primary are two Belmont residents, Margaret Hegarty and Bobby Reardon, and Dave Rogers of Cambridge. One Republican, Tommasina Anne Olson of Belmont, is also running for the seat.