'Don't knows' and 'None of these' have been combined.
Full answer wording as follows:
A referendum should be once in a generation, and Scotland should not
have another referendum until many more years have passed.
A referendum should be once in a generation, but if the political situation
changes then it may be right to have another referendum sooner.
A referendum is not necessarily once in a generation, and there would not
be anything wrong in having another referendum sooner.