The first thing that occurs here is that, although the upstairs owners have exclusive use rights to the roofspace, it doesn’t mean it belongs to them. In fact, it may well mean the opposite – the roofspace is common property but only they have the right to use it. The same principle applies to balconies in most modern apartment blocks.
So, yes, you can probably block the renovations unless they are done on your terms.
Those terms might include that the owners take full responsibility, individually and collectively, for any alterations to the the common property (a standard provision) and that all three of you agree to a redistribution of the unit entitlements so that you retain 26 per cent of the UEs and they have 37 per cent each.
You needn’t seek any compensation for the effective purchase of common property but they would pay all legal fees for setting all this up via an experienced strata lawyer of your choice (and I highly recommend either of our sponsors Sachs Gerace or Bannermans – see ads at the top of the page).
Failure to agree to any of these very reasonable conditions might well trigger your objection to the plan.
I’m sure there’s a nicer way of putting all that, but if you want to retain some level of control over the big decisions in the building, that would be my approach. And I would have a chat to a lawyer right now.