In NSW Schedule 1 part 2 refers to a motion being put to the meeting by a singular person.
So to be exact to the law, only one person can propose a motion.
Thst does not prevent multiple people proposing the same motion.
So in this case, the motion proposed by three people is not in accordance with the act ( though it would be a brave chairman to stand on this point).
And in this case, the withdrawal of one proposer of the motion would not automatically withdraw the motion, because the act considers that each individual has proposed the same motion , separate to any other proposer.