Tufts Medical Center and its physicians have reached a new three-year agreement with Blue Cross Blue Shield of Massachusetts, the two sides said. Tufts and its doctors' group said last month that they would stop doing business with Blue Cross on Jan. 17 because the two sides could not agree on a new contract. If no deal had been reached, tens of thousands of Blue Cross members would have been forced to change their hospital and doctors, and employers across Massachusetts would have had to consider switching to another insurer. The deal, in which Blue Cross agreed to give Tufts and the doctors a 3% annual increase in fees for treating the insurer's patients, means there will not be a disruption in care.