My question is can I stack kymeras in the hq using archons terror then paying 3 resources and putting 3 kymeras in hq.

No. A single reaction effect can only be triggered once per triggering condition.

Ktom, that is absolutely incorrect. The rules specifically states that multiple reactions can be executed off of a single triggering condition. Player with initiative gets to option to react first, then you take turns till both players pass. So yes, you may play an event and then trigger all 3 Beasthunter Watches off of it.

So yes, you may play an event and then trigger all 3 Beasthunter Watches off of it.

But that wasn't the original question. The original question doesn't say anything about 3 separate copies of Beasthunter Wyches. It merely asks if khymeras can be stacked by paying 3 resources to put 3 khymeras into play.

Sure, you can trigger 3 separate copies of Beasthunter Wyches 1 time each when an event is played. But the original questions asks if 1 copy of Beasthunter Wyches can be triggered 3 separate times when an event is played - and THAT is absolutely against the rules.