My copy starts in windowed mode and I couldn't find any settings to set it back to fullscreen mode.

I am running a clean copy of SC3KU.

Are you running the game in compatibility mode on Windows 10? Some people report that running the game in compatibility mode makes it start in windowed mode.

Have you tried to press ALT+ENTER to return it to fullscreen?

Edit: Also just wondering which OS do you have installed and did you made any changes to your pc recently or did you try to change the resolution of your SCU3 when this started to occur?

I tried Alt + Enter, no luck.

I am running Windows 10 in a multi-monitor setup. I was messing around with the hex edits to enable HD resolutions and suddently this issue popped up.

I did a reinstall and the issue is still there.

My guess is that, since the game is so old maybe it might be having some kind of conflict with your multi-monitor setup. But its just a guess...

Strangely enough I never ran into this issue and I have been playing this game almost everyday for the last 10 years...

Have you tried to play in another pc or disable your secondary monitor to see if this issue persists?

I've tried loading it without the second monitor, but I still get the same bug (windowed with a portion of the output cut off). My guess is the bug is due to using multuple monitors and the Optimus GPU setup. I can still play the game on my monitor, but its unplayable on the laptop screen. Guess I just need to get a proper desktop. :)

SC3K is a great game. A unique combination of strategy-type complexity and ease of use (relative to SC4). The game is almost 20 years old!

Hey I was messing around today with the hexdata in the SCU3 executable trying to understand better how it works in order to try to make more mods and I ran into the same issue of the game running in windowed mode that you are having. To fix it I had to unistall the game and install it again and then I patched it with a script I made in python sometime ago to play it in HD. Afterwards it went back to fullscreen normally.

Strangely enough if i try to play with the vanilla executable I cant play it anymore in fullscreen just like you. I can only play in fullscreen using the one i patched to play in HD. Dont know what happened.

Use a tool called DxWnd, it makes old DirectX games run in modern hardware, you can either force windowed mode or fullscreen with it, you can apply framerate-limit, and much more... Just make sure to enable "Expert Mode" on the menu and you should be ready to go. I'm making a guide on it soon...

dgVoodoo2 was the best option for me. The game looks so much better. Very easy to use.