I was trying to figure this out and was having a bit of confusion. What's the full range on card rarity, and to what extent can crafting play in this?

In particular, is there any practical use to having more than 3 of a card? I think (but may be wrong) that I can use the same cards in more than 1 deck. I've played other computer-based card games in the past where there were benefits to having more than 3 of the same card; is there anything to do with those extra cards worth keeping them around, or should I just scrap any extra cards after the 4th, and then use that to craft what I don't have (either now or in the future)?

I'm also slightly confused on the mythic cards, and the method(s) those come from. I found mentions that they can come from both the Pandora runs and also crafting, but not sure if this is stuff changing in the game over time.