Hi Kyle,
Glad you are enjoying it, and thanks for giving the games a shot!
> Are [Aces and 3's] up for grabs when anyone acclimates, and just not affected by "keeping things current"?
That's correct, either player can acclimate with any of the available Aces and 3's on a turn (not just the topmost in each stack), and they are not affected by "keeping things current."
> what are they used for, or how do you obtain them?
3's are played like the other numeric cards—they have an acclimate value of 3 and a cost of 3. Aces are mostly like face cards in that they have a cost of fifteen and are worth ten points at the very end of the game, but they have an acclimate value of one (this bit is in with the face card section under "Acclimating", but it's outside the context of them being set to the side).
Aces and 3's are not replaced by a card from the acclimation deck once taken by a player. Replacing cards only applies to the acclimation row (the five cards that cycle through when you "keep things current"). So all Aces and 3's are available from the start until they are gone but nothing will go into their spot once taken.
I hope that clarifies things; let me know if you have more questions or feedback!
Cheers,
Thomas