Against a, X gets to apply all defensive bonuses (assuming he knows the strike is coming).
Assume his shield is in his right hand and weapon in left.
He can use the shield against a. He could also use it against b or c as you can change your facing during combat, but his back would then be turned for other attacks.
Quickness always applies unless...
* He is surprised
* He is unconscious, sleeping or bound
He can parry b and use shield against a, but he then turns his back to c.
Instinctive Defense - +10 all the time, even when attacked by multiple foes. +20 if he knows that the attack is coming. If X knows that a, b and c are all armed and trying to kill him, then he gets the +20 against all three. If d comes up behind x without x knowing, then he only gets +10 against d, +20 against the rest.
Agile Defense - He gets this bonus against all foes similar to the QU Bonus, provided he knows they are attacking.
QU Bonus applies against all attacks assuming he is aware of them.