Other brackets beside 3s?
Beside that there is only 2s and 5s. 2s is already somewhat active on all wotlk realms and 5s will remain dead no matter if we have cross-realm or not.
3s is the bracket that could benefit the most from cross-realm but I don't see this happening when soloq remains a isolated bracket. The players from duoq and 3s won't be enough to make 3s active.
As I said, the point of cross-realm serves other things than JUST 3's. It is pretty clear. Unless you're wanting me to make a list as to what cross-realm and clustering does to benefit the game? Because that's something you should able to look up on your own.

Well, but thats actually what gonna happen. Malaco already said that duoq and 3s gonna be merged.

In case there is a confusion. By "merged" I mean that players from 3s gonna be able to face duoq players.
I said that wasn't the purpose of cross-realm. That is not the same as saying brackets won't be merged. That distinction is important because what you're saying I said is not what I said.