Are you in NSW?
My opinion – if the agreement is not in writing and if the correct legal process for changing responsibility has not occurred then it won’t stand up in court.
Verbal agreements are very difficult to prove. What evidence is there that an agreement was reached and what evidence is there of the terms of the agreement?
I am assuming from your question that the balconies are on the registered plans as being an OC responsibility. If the balconies are on the registered plan as being an OC responsibility then they remain an OC responsibility.
All three owners would then need to contribute to the repair and maintenance of the one balcony that is in poor condition, with the money coming from the Capital Works Fund and/or a Special Levy based on lot entitlements.
If you don’t know where the boundaries are then you can obtain a copy from NSW Land Registry Services.