U.S. stock index futures were higher on Tuesday, indicating a potential third straight day of gains as investors continued to bet that tensions in Ukraine would not escalate dramatically. Equities have come under pressure because of the problems between Russia and Ukraine, with the S&P 500 at one point down about 4 percent from a record closing high in late July. The gain implied by futures came despite ongoing concerns about the region, with Ukraine reporting that Russia had massed 45,000 troops on its border.