If she had more time, she ____ go away for a while.
There is something missing with 'should' that I cannot place my finger on.
The other two are equally possible but I prefer 'could' because it is a statement of fact where 'would' is indirect and unattributed.
Maybe she would whittle marshmallow sticks and play Tetris with the added time instead.
With "should" it should be a double conjunctive. Maybe "she shoulded". Or "she should oughta".