First of all, after acquiring those three True Gods, even a Divine King-level force would see a slight increase in power. Although the power boost wasn't much, at that level, any increase would be an opportunity that no Divine King would pass up.
Moreover, even stepping back, even if those Divine Kings thought it wasn't worth offending Zhou Tian for the sake of three True Gods, given Zhou Tian's situation at the time, they were absolutely confident in continuing their actions.
The reason for their behavior at that time was simple. Besides Zhou Tian, there was likely no one in the Divine Realm who believed his camp could achieve victory. After all, the main force in Zhou Tian's camp was the Divine King he had helped. Although Zhou Tian's war achievements were commendable, the fact that he hadn't truly engaged in battle at that point made it clear he wasn't considered a main member.
