You really cannot change the screen resolution in SH3. It is built for 1024x768 screens. However, there are add-in mods that offer widescreen in various resolutions, such as the
ARB mod, as an example. You could also download one of the 'mega' mods, such as GWX-KC, NYGM Iron Coffins or OneAlex, and those come with an included widescreen mod for 1920x1080. Living Silent Hunter has a couple of editions with different resolutions. There are stickie post for those mods in the
SH3 Mods Workshop forum