3 teams on 3 continents is not a problem. Moreover, when teams are separated strongly, i.e. no video and/or voice communication between teams, it helps to create healthy team, because non-realtime text-based communication causes well-written documentation and well-defined goals (just because it is much faster to write throughout description of goal once, in lengthy email or ticket, that to iterate with iteration cycle length equal to 1 day). Just use SCRUM, and follow it fanatically, and it will work. BTW: 2 week sprints are the best in such case.