Anon asked: How would it go if the girls were the one/ones dub-conning Zuko?
Azula will be the first to do it, obviously. Right after conquering Ba Sing Se, she is gonna want to "celebrate" their victory with him… and she'll want him to "keep her company" while she's bored as they sail home… and then she'll want a "truce" right after he barges into her room demanding to know why she lied to Ozai saying he killed the Avatar… and then she'll just want to connect with him somehow on The Beach… basically, Azula just wants him all the time, and he while he enjoys it, he's constantly trying (and failing) to resist her advances due to his guilt and shame.
As for Mai, once she is in that cell with him at the Boiling Rock, she might "lash out and punish him" for the way he broke up with her. Ty Lee will probably jump his bones the second she realizes he's boning both of her friends (which would take her about five minutes to notice), since she figures it's only fair they ALL get to sleep with him.
In an AU where Azula managed to capture Zuko right in the beginning of book 2, or where her friends did not "betray" her, they're all dub-con-ning him at the same time. In an AU where Azula wins the war, she's dub-con-ning all three of them.
Anon asked: How about scenarios where girls dub-con each other?
There could be some Mailee dubcon (with either of them in charge) while they were still obeying Azula, and either of the girls wants to try finding some kind of comfort with the other, and is a little too forceful.
Azula could dubcon her friends while they're at the boiling rock, and be dubconed by them after her breakdown.