How can I download Fotoslate 3 for Windows 64 Bit, because every time I try to download, it always changes to Fotoslate 4.
You can't download FotoSlate 3 because that version isn't available anymore. I've checked the official website, and it appears that only version 4 is available for downloading. The only way to obtain the version you need is through a direct contact with the developers. They should be able to give you a download URL if eligible.
Download page: http://www.acdsee.com/en/support/fotoslate-4
Contact page: http://www.acdsystems.com/en/about/contact-support