If you used a Dosdude patcher (you said "Capitan," but El Capitan works natively on the 3,1 so I assume you meant Catalina), then you probably cannot keep the startup disk as is. I recommend wiping all disks in the system and reinstalling OS X to a blank disk from a bootable USB installer.
Snow Leopard is not available unless you have an install DVD.
El Capitan is downloadable direct from Apple and you can make a USB installer to boot the system, wipe all disks using Disk Utility, and then cleanly install El Capitan. Read this page to download an El Capitan installer and create USB installation media:
You can use a USB flash drive or other secondary volume as a startup disk from which to install the Mac operating system.
support.apple.com
El Capitan is the last version of OS X to run natively on the Mac Pro 3,1 so that is probably the most desirable version to install if you're selling the system. If you really want Snow Leopard, you will need to find a disc for it. For security reasons, I do not recommend downloading software off of random sites online because you never know what you're getting.
Let us know how things go with your install.