Perhaps the 3m is the distance of unit area before you get to the common property. IE if it were longer it would go onto common property? Perhaps there was some general principle agreed to at some point for consistency of architectural style? It is not very nice that no one explained the reasons if there is something like this. Our OC has an Information Booklet that explains things like this: how far the unit area generally extends, what you need to do to get approvals, what styles of fences have been approved by general meetings and so on .