i suppose it depend on distance actually, a unit move is not a small operation, so theyd need time for the crew to travel, get intio their own digs, set up shots etc.
they also do a lot of in-car shots and travel shot which take another few days so the whole thing is not literally 3 days, but probably more like spread over 2 weeks in a block.
inbetween all that they may have to organise permissions and logistics to film in other venues, like pubs etc, if they havent in advance, and set those shots up...takes ages.
they also probably film a while in order to get the best footage - one day is not really enough to do all what they do...meals, days out, check everything, etc etc