What computer can I use for the DM3.1 to correctly download it for my PC?
The developers of the application don't offer support for DM3.1 anymore. Their new version is DM4, but it seems it's impossible to download it from their official website. Instead, access the download page at Software Informer to obtain the package.
Download page for Suunto Dive Manager: http://suunto-dive-manager.software.informer.com/download/