Board Thread:Versus Debates/@comment-24163532-20160313002413/@comment-26636534-20160313093145

Sasori most likely poisoned the Third somehow, before or after kidnapping him. I think that Sasori lased the Third's iron sand with poison and when the Third used it for battle, it was only a matter of time before he succumbed to it.

It is also unknown how Sasori managed to deal with the iron sand in battle. One method would simply be to go through a lot of puppets until the poison takes effect.