I use OBS, but i launch wc3 with edited settings, so it does not look like its windowed
Put a shortcut onto desktop, open properties, and at the part where is the path to the file ("D:\_HRY\_HRY_nainstalovane\Warcraft III\Frozen Throne.exe") <example
add the word windowed - "D:\_HRY\_HRY_nainstalovane\Warcraft III\Frozen Throne.exe" -window
after that, launch it via the edited shortcut, and y can drag the edges of it to match a fullscreen, but thats not all
open your OBS, and select - add window capture, then right click it and select properties
then launch wc3, put it as the captured window, and tick the inner window box
that will cause the wc3 to appear fullscreen on stream, but you ll still have to play it windowed
i used to do this, but now i dont stream wc3 - little audience, and takes alot of time