What should have happened:
The Parish Council advertises the vacancy for co-option on the village noticeboard and ideally website. Details of your application are circulated with at least 3 days notice to councillors. (Though the full agenda should have included a section to confirm a co-option hearing).
You are interviewed, or your application is read by councillors.
Where multiple councillors are to be co-opted, the proceed in law is very clear, they must be elected by a majority (50%+1 of the councillors present, not just a plurality). e.g. 10 councillors present, 5 vote in favour, 3 against, 2 abstain, then you are not elected.
It should be done by a show of hands, unless a councillor specifically requests a secret ballot.
If councillors wanted to co-opt all three, they could have put a motion to that effect to override the voting procedure. But in order not to select you (after considering your application) required a vote. - You can ask the Parish Clerk to confirm the voting outcome.
