Using what appears to be the Arena ID found in the HTML for the images, Scryfall was able to put the cards in what is effectively collector number order. In doing so it appears as if the contents on the 3 bonus sheets are now clear. I’ll list them below in the order they’ll run rather than the order they appear in the Scryfall link above.
“Planeswalkers”, which will run from December 10 – 24, are the cards from Ajani Steadfast to The Chain Veil
“Spells”, which will run from December 24 – January 7, are the cards from Hallowed moonlight to Wear//Tear
“Devotion”, which will run from January 7 – 21, are the cards from Eidolon of Rhetoric to Xenagos, God of Revels
While this is not officially confirmed, I would say this has a very high likelihood of being correct as each set of cards fits at least loosely into the categories and contains 40 cards each.
