You need to bear in mind that an owner has to consent to an exclusive use by-law that binds them. So in this case the other two owners enclosed their balconies with the approval of the EC, but without a condition that a by-law be put in place. Ideally, that should have been a condition, but it wasn’t. You can’t really blame those owners, it was up to the EC. From what adammark has said, the owners didn’t do anything wrong, they sought approval and they got it, the work wasn’t illegal.
The EC/OC could go back to those owners and say they now want to put in a by-law to cover those balconies, and the owners could refuse consent. If they did that, then the OC has limited recourse although they could try and compel them through the CTTT, not sure how that would go.
So now the EC has changed and the policy has changed. The previous approvals aren’t really relevant, and I don’t believe that saying the other owners didn’t have to have a by-law will help in getting approval. If you want to get approval, then agree to the by-law as it is not an unreasonable thing to ask. At the same time, suggest that for consistency’s sake it would be a good idea to have a by-law covering all three balconies, spread the costs and hope that the other owners consent to the by-law.