Where can I download Heroes WOG 3.5, 3.x?

The simplest solution is to buy a licensed copy of The Shadow of Death, download patch 58f (I have links in my signature), and install it according to the instructions – I think this will solve your problems. If not:
try uninstalling your sound drivers and playing without them.
you can also try the same with your video drivers – change them.
if that doesn't help, contact me on ICQ – 375572427 – we'll figure it out together.

How can I prevent the main1.wog and other files from being corrupted?

try following the instructions – I also encountered this problem during the first installation. I only remember that the archive is not the issue – all the files are intact.

It won't install on Platinum! It only agreed to install on Windows XP, and even then, it doesn't work.

patch WoG 3.58f installs on:
licensed Heroes 3 Platinum Edition
licensed The Shadow of Death
other versions of WoG