Murder husbands Marco and Sabo, they are so in love with each other but with their intense and dominant personalities they always have some friction with each other, that's why they always have a third person to sooth it. (Also important note, usually the victims are horrible people, who aren't punished by the justice system)
However noone lasts in that position, sometimes they seduce someone slowly, but when the person finds out what kind of monsters they are, they try to escape, which leads to captivation, but that hets boring for Marco and Sabo pretty soon, so they get rid of the person. When they want to have some short term, quick fun they just straight out kidnap someone, then get rid of them. A person lasts from one month to four at most.
But Marco and Sabo want to find the one. The one who fits them perfectly and they are not afraid to look for them.
One night they see Ace at a park where he's walking Kotatsu (still a lynx, Ace has everything he legally needs to keep him), the sight makes both Marco and Sabo interested.
The next day they make sure to find out everything they can about the man who was walking a lynx at the park. The methods include (but not limited to) asking around, bribing, threatening, stalking. When they are satisfied with the results, they start part 2 of the mission, approaching Ace.
After Ace finishes his work and goes home, Sabo "accidentally" bumps into Ace (who's busy with typing on his phone), spilling his coffee onto his own jacket, making Ace embarrassed and apologetic. Sabo ask him compensation in form of accompanying him to dinner, there is a little hesitation but Ace eventually agrees. They go to a somewhat fancy restaurant, and Sabo insists that Ace stay for dinner, it's his treat, and Ace finds Sabo hot, so why not. They are led to a private room where Marco is waiting, and when he sees his partner accompanied by their future lover, he can't help but smile.
Ace is confused, he thought the man he bumped into wanted to seduce him for a night and now he's having dinner with Sabo and his husband Marco, without any sign it'll lead to more than a friendly dinner. He's slightly disappointed but they are so nice and kind, he quickly forgets about it.
The dinner ends with a good atmosphere, and Marco and Sabo takes Ace home, somehow tricking him into agreeing to meet them again.
The pair makes sure Ace is cherished, taking him to dates, giving him small gifts, listening to him, helping him in everything they can, and Ace never had anyone care for him like that (he has Luffy, but Luffy is the younger so it is Ace who takes care of him) and it feels so nice, he can't help but fall in love with them.
It takes six months to get Ace officially join their relationship, but both Marco and Sabo are satisfied, because Ace is everything they ever wanted from their third; kind, beautiful, compassionate, strong and fierce and full of fire. The only issue is their not so acceptable hobby of murder.
They hide it as long as they can, being more careful than ever before, not wanting to scare Ace, because they'd have to get rid of him if he's reacting badly.
In the end Ace finds them torturing some random dirtbag who harassed young girls, and while it takes some time, he accepts them fully and sometimes even watches them serving justice in their own way.