It’s a turnover in all three cases.
There are 2 important points:
1) The thrower can never call a violation to get the disc back in these situations. Rule 2.7 states:
The immediate surroundings of the playing field shall be kept clear of movable objects. If play is obstructed by non-players or objects within three (3) metres of the perimeter line, any obstructed player or thrower in possession may call “Violation”.
So the thrower must call the violation before the throw (‘thrower in possession’), when people or movable objects are in the way, and can pause the game until those people or objects have been moved. You can’t get that disc back after throwing it.
(Note that ‘obstructed player’ means a player who is physically impeded – e.g. getting tangled in a crowd of players after making a bid near the sideline.You’re not an obstructed player just because the disc you threw was obstructed.)
2) The first line of that rule refers to movable objects. It’s reasonable to assume that the second line does also – certainly that is my interpretation. If it applied to a tree, then what would be the resolution of a violation called before the throw? We’d have to get someone to chop the tree down before we could play on. 🙂
If the field is considered safe and unobstructed under rule 2.1, and you’ve chosen to start the game, then there’s not much you can do during the game if the disc hits a tree. It’s a turnover.
Note also the definition of ‘Ground’, which includes trees etc and makes clear that touching a tree is the same as touching the ground, and the definition of OB which states that everything other than the playing field itself is out of bounds. Tree branches, or indeed a roof, are out of bounds even if directly over the pitch.