Another player came up with this on cardgamedb rules forum, and we couldn't reach a conclusion:

Queen's Guard:
"After you trigger an effect that kneels an opponent's character, choose and kneel 1 character controlled by a different opponent."

+

Harry the Riverlands:
"Kneel any number of characters with combined printed cost of 3 or lower."

Queen's Guard says "...kneels an opponent's character..."
So it doesn't apply if I kneel more than one with one triggered effect?

Queen's Guard says "...controlled by a different opponent."
So I can't kneel all the Refugees of all my 3 opponents and still trigger him, as it has to be an opponent whom I didn't kneel anything of with my triggered effect?

Thoughts?

It has been ruled by FFG that "different opponent" is from the perspective of one character just knelt. So if you kneel characters controlled by multiple opponents with one effect, you can kneel a character controlled by any opponent.

Note that Queen's Guard's ability is passive, not triggered, so it initiates whether you want it to or not.

Thats good to know.

What about

an effect that kneels an opponent's character

Is that defined as an effect that kneels one opponent's character, or can it also be defined as any effect that includes as part of it the kneeling of an opponent's character? That is, is kneeling three opponent's characters sufficient to trigger this?

The comparable precedents here are that searching a portion of your deck doesn't count as searching your deck, and that discarding your hand isn't the same as discarding a number of cards equal to the cards in your hand. Thus it may well be possible that "an effect that kneels an opponents character" must be one that does exactly that: just an opponent's character, not several.

My response on cardgamedb was the same as yours (I'm there as Asklepios), but we're not sure.

Also, do you have a citation on your FFG reference? Thanks for the trouble!

Consider Starfall: "Response: After you win a challenge in which a House Dayne character participated, place 1 gold token from the treasury on that character." You can trigger that even if more than one House Dayne character participated in the challenge, right? If there were multiple HD characters in the challenge, you get to choose which one you are "referring" to when meeting the play restrictions and placing the gold token.

Same thing for Queen's Guard. The effect that knelt characters which allows the passive to initiate can kneel more than one character. (And notice that it is the resolution of the effect, not the change in status of the character, which allows the passive to initiate.) When resolving the passive effect, you decide which character is being used to satisfy the "kneels an opponent's character" restriction. The character knelt for Queen's Guard will have to be controlled by someone other than the controller of that "reference" character.

For Example: Player A uses Harry the Riverlands to kneel Refugees controlled by Players B, C, and D. Player A then resolves his Queen's Guard, choosing the Refugee of Player B as the "reference" character to satisfy the play restrictions on Queen's Guard. He can, therefore, choose and kneel a character controlled by C or D (but not B, the controller of his "reference") with QG's passive.