Hi, I'll answer your question below to avoid spoiling anyone!
.
.
.
.
.
.
.
There are only two choices that matter for Ending 3, and the first one of those choices you might not even encounter during your playthrough, in which case there is only one choice that matters. You only encounter the first choice if you kiss Ella. After the kiss, you must answer:
"No. I won't leave them."
After that, the only important choice left is this:
"I'd like you to stay."
I hope that helps!