This is my first time giving advice in this sub so bare with me if I say things that might be a little too extreme. It's very unfortunate your own boyfriend has to see you having sex with 2 other guys. I believe this scenario can relate to when an SO cheats, the victim usually gets depressed & angry. Unfortunately your boyfriend is only depressed, yet he can't get angry at you considering this was before you dated him. Reasons why his mood and personality changed can possibly be for these:

-Seeing his own girlfriend, the one he loves, naked, and pleasuring two males -knowing his girlfriend is the type of woman to have sex with two guys at the same time -Jealous for allowing your ex to have a threesome and tape it. -He wants more detail about the video and how it happened, though he can't 'cause it will hurt him more

My advice? Make him forget about it. Be more passionate with him, make him feel like he's the best one that you've slept with, even bluntly telling him he's the best sexual partner for you and that he makes you forget about every guy. Hell, even offer (jokingly) a threesome with a female to make him more confident about himself. Talking about it can only do so much, it's the actions that will speak louder than your own words.

Another advice I can give, though I won't recommend because it involves lying, make up a story about how it happened. Tell him you were forced into the threesome, or your ex threatened you, or something that will make him believe whatever happened in that video was something you hated and was obligated to do. Then he won't feel bad about it happening in the first place. As long as you never cheated, maybe lying and risking his trust for you can be a good sacrifice in order to save the relationship (IF that video affects his current mood permanently).