Did you see those bigass blocks of IS he created? All that cound not fit in that small puppet. Sasori could increase the amount of IS drastically very fast. I guess that 3rd Kazekage was similar to Rasa, Gaaras father, who could produce large amounts of Golden Dust in no time wharsoever. Keep in mind, that 3rd was said to be considerably stronger than him. And if 3rd could do it, so can the puppet that was made of him, since thats how human puppets works.
*sigh* This makes me wish we actually saw the Third Kazekage rezzed in the war.
I concede it's
possible the puppet could naturally produce or summon Iron Sand, which would put the Third Kazekage at mid diff instead of low diff. Sasori said he poisoned the Iron Sand that the puppet used, so that was another hint for me that there was a limited amount at his disposal, and there isn't a way he could reliably say that if he could just make more of his own.
On the other hand, the Iron Sand could just be produced or summoned inside the Kazekage's puppet body and doused in poison on the way out, I suppose. That still limits Sasori's options with the Kazekage. Even with a limitless supply of Iron Sand, Gaara has much more true sand readily available than Sasori has to prep and funnel out of his puppet.
If Gaara battles aggressively right from that start against the Kazekage puppet when it comes out, Sasori is still forced into a defensive position, since the puppet is still disabled once Gaara can lock up the Kazekage's mechanisms with sand. I reasonably assume Gaara has intel on the Third Kazekage and his abilities. I'm still in Gaara's corner on this.