The 3rd sacrifice appears to have been a red herring. The LL Q&A clarified Venat's primary concerns were the Ancients becoming The Plenty and not being able to defeat Meteion due to their inability to interact with dynamis. My theory is it was likely the pretense she used to amass followers. There's no indication she ever spoke to anyone about what her true motives were.

I feel it important to note that Venat consistently shows no care for souls or even the lives of those outside of the Source. As far as we're told, the 3rd sacrifice would've been the last regardless of what was being sacrificed. The sundering ultimately didn't stop the Final Days from returning in a substantially worse form and seemingly permanently destroying the people who became blasphemies. Her maintaining the time loop also meant that 7 rejoinings + the Flood of Darkness + the Flood of Light had to occur. While I understand the concerns about the 3rd sacrifice, Venat's methods led to exponentially more death in the long run.