If you were in the ACT, I’d point out:
1) an EC can have three members.
2) remaining members of the committee can appoint another member of the OC to fill a casual vacancy (but not obliged to do so).
3) While it is usual for the positions of secretary, chair and treasurer to be held by different members of the committee, that is not required. One person could be any two or all three. The committee elects those position holders.
People in other states would need to check things are the same where you are.
So, with or without the 4th person who might be interested, I don’t see why the remaining members of the committee could not hold the fort and instruct the strata manager as required until the next AGM.
BTW. Why did the last two resign, the ones who had not sold? Were they not getting on with the three who stayed or two or perhaps one of those who stayed? If you have one person who is pain, the other two should stay, if only to keep them from doing too much harm.