Complex sentence: For a period of 3 weeks, there were only be 2 crew members; it was the first time that had happened since STS-114 had delivered a third person to restore the ISS crew to 3.
Split into simpler sentences: For a period of 3 weeks, there will only be 2 crew members. This is the first time this has happened since STS-114 delivered a third person, to restore the ISS crew to 3.