Cushman & Wakefield has completed three office lease transactions totaling 37,060 s/f.  The recent transactions signify activity in the leasing market despite these economic times. *Sage Software, leased 12,400 s/f at 100 Elm St., a facility owned by Historic Realty.   *New York Life Ins. expanded by 10,856 s/f at 690 Canton St. in Westwood.  The landlord, Colony Realty Partners, was represented by Nat Kessler of CB Richard Ellis. J.P. Plunkett and Catherine Minnerly, C&W senior directors, represented Sage and N. Y. Life in these transactions. *Altra Industrial Motion leased 13,804 s/f of office space at 300 Granite St. in Braintree from Sun Life. Altra was represented by John Dolan of Colliers Meredith & Grew, while Sun Life was represented by Plunkett and Minnerly.