Must Gather/Push effects which don't say "up to" move exactly that many things? Eg: Does Fields Choked with Growth move exactly 3 Dahan?

Must Gather/Push effects which don't say "up to" move exactly that many things? Eg: Does Fields Choked with Growth move exactly 3 Dahan?

Yes, if possible. If a card gives a number of pieces to move, you must move that number if you can, or as many as possible if there aren't enough of those pieces there.
Most "move some Dahan" powers represent the Spirit asking/working with the Dahan or calling in favors. A few, like this one, are more "the Dahan decide to get the heck out of Dodge". This applies to other kinds of pushes, as well.
Other cards with similar wording are Terrifying Chase (Sharp Fangs behind the Leaves), Sacrosanct Wilderness (Keeper of the Forbidden Wilds), and Twilight Fog Brings Madness.