I was gung-ho about seeing this season after having purchased and absolutely LOVING the first two. That said, I need to make it clear here Season 3 does not live up to the previous 2. Maybe it is just the edge was missing.
Earl season 3 was not by any means terrible, definitely worth seeing once or twice, but it was far from the barrel of laughs that seasons 1 and 2 provided.
If you are an Earl fan and NEED to get the whole collection, definitely buy it, but for most people, I think that renting season 3 will more than suffice.