Before the Columbia accident we always had expeditions of 3 that would exchange with 3 new astronauts on the same mission. Now we always exchange first two astronauts and then a month later or so the last astronaut. Why is this so? I know that just after the accident we only had two astronauts on ISS, but since we got back to full 3 crew astronaut occupation we always exchange two astronauts at a time and a month or two later the final one would be changed with a new astronaut.
Is there a reason for this? Did we find a little advantage in exchanging the astronauts like this?