in your example, A and B are affiliated with each other through X.
there is no direct conection between A and B, therefore no 3x exp.

if you wanted 3x exp, you should have done, X refers A, A refers B.


and yes, as a repeat, the second example of having a booster on account A will work, since X and B are directly connected, and are within 4 levels of each other.




IF this is how you set up your RAF, X->A; X->B, then you should level all three together, via questing. once you get a character on either A or B high enough to run other characters through instances, then you can utilize the instance boosting method.