Steve Grigg
These Longhouses are most likely from the medieval period. Longhouse 1 and 2 are shown in the Jeremy Butler Dartmoor Atlas of Antiquities, the North (volume 2) map 30 and are close to the Middle Blowing House (of three) up the River Walkham valley from Merrivale Bridge. Longhouse 3 is close to the Lower Blowing House just 300m up from Merrviale Bridge and in this post is just a personal speculation that it is a Longhouse.