@Lonewolf685
If there really is no chance of them not betraying the Machina, then just use the opportunity to deal with all of them permanently. Drop their guard in any way possible and wipe them out.
By the very nature of the Nexus that's not possible given death holds no permanence, unless Eos wants to go down and try to repeat Artoria's eradication on them, though even she might not have it in her for all three.