If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and Excalibur II's first ability triggers only once
Note that the creature with Coven doesn't have to be part of those three creatures for its ability to trigger, it simply needs to be on the battlefield alongside those three
Consider Treacherous Blessing and similar ETB enchantments for maximum value
While its far from the most consistent deck archetype in Pocket (due to needing two different energy types), Dragonite is an eminently strong Stage Two mon with an absolutely staggering attack: Draco Meteor, which deals a towering 200 damage (albeit in a random fashion)
And if you were to hold onto a pack or two (or 10) for yourself, who's to know or care
Even though Sylveon backed off from this price, it later ran past the $100 mark to rise to its current market price