With three dying at once without warning, I would suspect either poisoning (food, bedding or hutch material) or a big fright by a predator trying to break in if they were in an outside hutch.
Give your survivor the best of care - he may be in shock - while you find him a new friend. Have you been to the vet with him to make sure that he is OK?
We had a case earlier this year where two boys sadly died from their bedding. and one case where they had eaten something on the lawn that was poisonous (like creeping buttercup).