Huey's ending requires you to finish Huey's storyline. This just entails you doing gold-marked Side Ops, namely the one(s) involving the AI Pod.
Quiet's ending is similar, but to get the last two missions to end her story you have to have the maximum Bond rating with her and do Mission 43. If you have the "Butterfly" emblem part equipped as the emblem for Diamond Dogs, then her ending will be stalled until you remove the "Butterfly" emblem. You must continue using the "Butterfly" emblem part to prevent Quiet's ending, otherwise it will happen after a chain of unstoppable events next time you autosave without the "Butterfly" equipped.
- From what I have observed of other in game events, if you complete
mission 43 without having max Bond level with Quiet, you can re do
the mission again later when you have max Bond level and it will
work. For instance you can choose to kill Quiet during your first
encounter with her in Chapter 1, but if you re do the mission and
don't kill her then Quiet will join your company like normal. Someone
correct me if this event has some kind of weird exception.
"True" ending is kind of vague, but I assume that you are referring to Mission 46. You have to complete all gold-marked Main Ops, Side Ops and listen to all gold-marked cassette tapes to unlock Mission 46.
There is another ending that was datamined from the game's files, and obviously unfinished. Google "MGSV disarmament ending" if you want to know more about that.
None of these endings are mutually exclusive since the game is structured in a sandbox like MGS:PW.