Highland Shade said:
Based on errata and timing of "response" effects, anything which triggers more than one response simultaneously can only happen once, not multiple times. So in this case, if a card/effect raises each players threat by 1 (simultaneous effect) the Song card can only be used on one of the four players.
Here's that part of the FAQ:
(1.08) Responses per Trigger
If a response or forced response is triggered, the effect can only occur once per trigger.
Example: Theodred (CORE 2) reads, "Response: After Theodred commits to a quest…." This effect can only be triggered once each time Theodred commits to a quest. (p. 4)
This would clearly mean that one response can be used once per trigger. So, if you have 1 Song of Earendil, you can only trigger the response one time by that card (you can't use that one trigger to continually activate the Song's response text). As I am reading it, though, this text doesn't address multiple copies of a card, or multiple responses being activated off of a single trigger. So three copies of Song of Earendil should be able to activate their response from a single trigger (but only one time each, as per the Theodred example above). Or, for example, if a character leaves play, you may trigger the Horn of Gondor's response AND Prince Imrahil's response AND the response on Misty Mountain Eagles.