For instance, can both me and my friend have 3 copies of Test of Will in our deck? Or is 3 the limit for all decks in play? Thanks!
(unfortunately I only have 2 core sets, so this scenario is hypothetical)
Can multiple people have the same 3 cards in their deck?
The 3 copy limit is per deck. So in a 4 player game you could have 12 Test of Will total, 3 per deck.
Yes, even if they are unique.
You can even play multiple copies from multiple players, as long as they aren't unique.