my guess is that the clones can play catch with the "destructo disk". Thus the reason for the three clones is so they can get him off balance and hit him with it.

Also I'm starting to think that while Hidan is clearly beaten that another Akatsuki likely the leader will show up to retrieve the head. Considering all of them were just together sealing the 2 and 3 tails it would stand to reason that he could be in the area. Plus from what we have seen of him passing through the ground wouldn't be an issue.