Maybe the three now mortal gods want to find a way to godhood that does not limit their ability to interfere in Faerun. They were made mortal because they did not stop their interference. As the Mindflayer gods are outside of the normal pantheon, could they seek to cooperate with them to become a less limited kind of god with no supreme entity to control them? This seems more like a story for a planescape setting, but the motivation is there for them.
I sometimes use thought experiments. I don't necessarily believe in every idea I post for discussion on this forum
