Complex sentence: Bad weather broke up the flight, with five B-17s returning to Labrador and 5 continuing on to Greenland, but three were forced down, including "My Gal Sal".
Split into simpler sentences: Bad weather broke up the flight; five B-17s returned to Labrador, while the remainder continued on to Greenland. Over Greenland three of the aircraft were forced to land by the weather, including "My Gal Sal".