There’s kind of a standing joke in some Courferre fics about who would get custody of Enjolras if they broke up.
But imagine things getting tense between Combeferre and Courfeyrac, and Enjolras being genuinely terrified because although they’ve both said that they love him and whatever happens between them won’t change that, it will. It will mean the end of them doing things together as a three; the end of the comfortable companionship between the three of them. It will change how they work, even if it’s just awkwardness between Ferre and Courf. If they won’t even talk to each other, Enjolras doesn’t know what he’ll do.
But he can ignore how it will effect him, because in the end, if Combeferre and Courfeyrac break up, it’s going to be hell for them. They hate hurting each other but if there’s no way round it then what choice will they have? Enjolras will look after them as best he can, and he knows that their other friends will, too, but it’s just – it’s going to be hell, and sometimes Enjolras wishes they’d never got together in the first place.