She seems to be on their level, and her interest is the same so more like she will be a 4th teacher.
Lets be real for a sec, they are training him to be stronger than them, they want him to be dominate, there is no chance they get to monopolize him if they get him to where they want him. There goal, even if they don't realize it themselves is to be in a harem, so I doubt she will suffer really, she clearly can escape them no problem.
yeah - the Original Three and the Thief are basically the "unstoppable force / immovable object" paradox, as Seras is basically them, just as a Stealth Build, rather than Strength / Magic.
If the Trio can't best one another, they're not besting Seras, either.
Especially since I suspect Seras has a cool enough head on her shoulders to know what to say mid-fight to taunt the Trio into infighting, as well (they do that well enough on their own as it is).
Seras will simply become Cross's fourth teacher, doubly so once the Trio sees what Cross is now capable of (I'm betting they'll stumble into this ball/oncoming fight with all these Underground people, and they'll get a front-row seat to what Seras has trained Cross to be able to do).
And however it happens, they'll ultimately be convinced by Cross that Seras isn't bad, that she'll make an effective teacher along an axis of growth that the Trio can't give Cross, and that they'll have to play nice to whatever effect.