step 1: if one of them is catherine, and she's pleased with you, bribed her to attack somebody. she's not loyal at all, she can be talked into declaring on someone she's friendly with. even monty won't do that!
step 1a, if step 1 fails: hope that they like me more than they like each other, tech towards mass media, and build the UN

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as far as real answers ...
try as much as possible to not declare until you think you can wipe out your target of choice during a single war. one "you declared war on our friend!" per dead guy is bad enough.
sometimes the AI will have "stop trading with" redded out but "declare war on" isn't. sometimes it's the opposite. if you can convince someone to stop trading with somebody else, then you only get the "you started a trade embargo against us!" with one leader, and it hurts the relationship between those 2 civs at least a little bit.
look at why they're so friendly. if they're running different religions, probably some of them are getting favorite civic modifiers. often it's a case of somebody running Organized Religion, which gives them a + with brennus for example, but it's not
their own favorite civic. you might be able to bribe them into running a different civic in that category for a while. that can often do the trick by itself. that "you have wisely chosen your civics" can be as high as +6, and will go away instantly when they adopt a new one. you might need to give them techs to have civic options. i recently gifted churchill Liberalism, so that he'd go to free religion. in my case it was so that he'd lose a -heathen modifier with hatty so that i could trade with her; but in the brennus case it would make him lose + civics for running OR.
a not-likly but ideal situation would be if civ B is far-ish away on the continent, and weak, but has a DP with civ A that you want to kill off. when you declare on A, B will DoW you. you'll get a - "war on friend" for anybody who likes A. but anybody that likes you will give B a - "war on friend" when
they declare war on
you because of the DP. of course you have to defend yourself against B if they do put up a fight. since you should always be checking for DPs before you start up wars *giggle*, here's a way you might can use them to your advantage.
i can't guarantee that any of this stuff will work of course. it's not gonna get them to go war against each other, but it might get you a few less "war on our friend"s if the stars are in alignment.