I'm missing Danny's third CG, and I have already seen all of his endings. How do I get that missing CG?
It has been a while but I believe Danny's 3rd CG happens towards the end of Chapter 4 and requires a specific "affection" score. If you managed to get the best ending without unlocking this CG you probably barely missed the required stat.
I would recommend (if you haven't already) trying to follow a guide that helps you get your score as high as possible through this area so you will be sure to unlock it.