The first exhibition of American paintings in the Soviet Union since the general-exchange agreement of the 1985 Geneva summit opens Wednesday at the Soviet Academy of the Arts in Leningrad, featuring the work of three generations of Wyeths. "An American Vision: Three Generations of Wyeth Art" features 115 paintings and watercolors by the late N.C. Wyeth and Andrew and James Wyeth, and it was organized by the Brandywine River Museum in Chadds Ford, Pa.--the Wyeths' hometown. "When we were over there negotiating the contract, everyone said there would be lines around the block," said Brandywine Museum director James H. Duff. The exhibition will remain in Leningrad for four and a half weeks and in Moscow five weeks. The Soviets are to send the Brandywine museum a reciprocal exhibition in 1988 or 1989.