A:
This ballot is acceptable.
Those choosing the third option are, in effect, abstaining from voting (i.e., their vote will make no difference to the outcome).
Usually, a ballot has just the YES/NO option.
A person wishing to abstain would mark neither, then cast his ballot with the rest.
The tellers would set all unmarked ballots aside, and not count them in any way.
They would also set aside the MAJORITY votes, except those votes would finally be added to the winner's side.
The outcome would be the same as if they were abstentions; it's just that the margin would be larger, thus making it appear there was greater support for the motion than there actually was.
Although this kind of ballot is acceptable, I can see it serving no honest purpose, and urge against its use.
Most votes can be Reconsidered within a short time (for most meetings, until adjournment).
See RONR Section 36 for details.
For votes that cannot be Reconsidered because the time has expired, consider the motions to Rescind and to Amend Something Previously Adopted, found in section 34.