My forum now is with V5 release 3, but now I can't find the setting that turn to Fluid View.

I have all type of forums, 3 leves, currently is with Card view. I have all main forums with Fluid View enabled, in this system I can see that description. But navigating my forum it only displayed as card.

O I have to change all subforums 100% to Fluid View too?

Theme editor , global , body set to the width you want.

This would be in Theme Editor > Page Layouts > Forum Index to determine what layout the interface for Forums would use.