The carriage house at 76 Downer Avenue in go-go Dorchester used to service the main house at the same address nearby. It was recently renovated, and is now one of three condos in a three-unit association.The 1,758-square-foot 2-BR, 1.5-BA has a lofty single-family look inside, complete with clean, sleek lines throughout. To us, it’s a nice example of modern design in a neighborhood otherwise best-known for gorgeous architectural throwbacks.How much to live well where horses once did their business? $769,000. That is $30,000 lower than when 76 Downer was originally listed a little more than three weeks ago.