Who's inside the third coffin that Orochimaru tried to summon? In the battle between Sarutobi Hiruzen and Orochimaru during the Chunin Exams, (on the rooftop where the Sound Four sealed them in a barrier) Orochimaru summoned the two previous Kages. He also tried summoning another coffin but Hiruzen successfully stopped it. 

Who was inside that third coffin? It can't be the Fourth Hokage right? Because that time, he was still under the Shiki Fuujin Seal.
As can be seen on this image, it was indeed the 4th Hokage that Orochimaru tried to summon.

  

The kanji that appears in that coffin is 四, which is the kanji for 4.  

However, unlike you said in your question, Hiruzen did not stop this coffin. It was the fact that the 4th Hokage was sealed with the Shiki Fuujin that prevented that summoning.