WASHINGTON (UPI) -Justice Sandra Day O'Connor, whose nomination to the nation's highest court was opposed by anti-abortion groups, Wednesday left no doubt that she adamantly opposes abortion. Dissenting from the high court's rulings in three cases, Mrs. O'Connor, 53, squarely backed state limits on abortions and used her chance to write against the majority view as a platform to criticize the landmark 1973 Supreme Court ruling legalizing abortion. Mrs. O'Connor, the first woman on the high court, disagreed with most of her colleagues who declared an array of state restrictions on obtaining abortions are unconstitutional. Writing the dissent in a case from Akron, Ohio, she said the legislature is the "appropriate forum" for deciding such' 'extremely sensitive issues'' as abortion. Such regulations as requiring abortions after the first three months be performed in a hospital are not "undue burdens" on a woman's rights , she said. The majority, however, disagreed. Nominated to the court by President Reagan, Mrs. O'Connor provided one of only three votes the administration gained in its campaign to persmtde the justices that legistatures should make the final decisions on such politically controversial questions.