Photo by Amberture The historic Randall School property in Southwest D.C. — which art collectors Don and Mera Rubell purchased for $6.5 million from the Corcoran Gallery of Art back in February — will get a new look, courtesy of three designers. The Rubells and the Telesis Corporation, who are developing the site together, have hired Bing Thom Architects for the principal architectural work. BTA is best known, or will be best known, around…