At about 13:34 EDT, a Cessna 305A and a Burkhart Grob G-103 glider collided in mid-air near the Philadelphia Gliderport, killing the pilot in the Cessna as well as the instructor and student in the glider. The Cessna was in a climb towing a glider, while the Grob was maneuvering. The pilot who had towed the Grob aloft said they had taken off at 13:25 and the Grob released at 2,500 feet agl. He returned to the gliderport and landed without seeing the Cessna. The pilot of a single seat glider that was being towed aloft by the Cessna said that they were climbing through 1,000 feet when he spotted the Grob at his 11 oclock position, about 1,500 feet away, and about 100 feet above his position. The Grob was (coming toward us) in about a 15 degree right angle of bank turn. As we continued, it became apparent to me that the flight path of the Grob and our flight path (single seat glider and the tow plane) would cause a collision if no evasive action was taken, he said. About 1,200 feet AGL, I pulled the rope release and turned to the right in about a 45 degree angle of bank. After about a 60 degrees heading change, I leveled off and looked to my left in time to see the Grob and the Cessna approach and collide. I do not think that either aircraft was taking evasive action.