How do I pick-and-match licenses via Family Sharing if multiple friends own the same game?

Steam will grant you access to a game via Family Sharing from the first (alphabetically, by Steam display name) owner of the original game. So if Foo and Bar both own the game, you will only use Bar's family share and disregard Foo's ownership of the game as if he didn't own it. Also note that you will only have access to Bar's DLCs. If Foo has some - you will not know of them. If you yourself buy the original game, you will lose access to Bar's DLCs.
Given that you have access to all accounts, my obvious suggestion for you is to change Steam display names to list them in 'most frequently used' - 'last alphabetically'. e.g.:

• Least used account: Ann
• Medium used account: Bob
• Frequently used account: Charlie

This way Ann will get access to Bob's copy of Terraria whenever Charlie is playing.