Here I have installed SH5 and SH3 on Windows 8 and everything works well.For SH3 there's no need to run it in compatibility mode(in the case it's needed run it in compatibility mode for windows xp service pack 3).The only important thing is that you run it always as "Administrator" and that you place the main SH3/5 folder in a place where win 8 permits you to save(here I have them in C:/Programs (x86)/Ubisoft).
Do not place the folder in C:/Programs or win 8 will not permit you to modify files in the future in the case it's needed.
Hope it helps!