18.04 - Ubuntu default `sources.list` causing `sudo apt-get update` to fail

I've been having some issues with my packages and repositories, so I decided to try and reset my sources.list file. I did so by removing it from the command line, creating a blank in its place, and using Ubuntu's Software & Updates GUI to try and remake it. This however, seems to have created new problems. When I try to run sudo apt-get update, I get the following output:

Get:1 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Hit:2 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Hit:3 https://brave-browser-apt-release.s3.brave.com bionic InRelease
Hit:4 http://packages.microsoft.com/repos/vscode stable InRelease
Get:5 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Hit:6 http://repo.steampowered.com/steam precise InRelease
Ign:7 http://ppa.launchpad.net/mc3man/trusty-media/ubuntu bionic InRelease
Hit:8 https://download.sublimetext.com apt/stable/ InRelease
Hit:9 http://ppa.launchpad.net/papirus/papirus/ubuntu bionic InRelease
Get:10 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [260 kB]
Get:11 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [517 kB]
Hit:12 http://ppa.launchpad.net/plushuang-tw/uget-stable/ubuntu bionic InRelease
Get:13 http://security.ubuntu.com/ubuntu bionic-security/main i386 Packages [197 kB]
Get:14 http://security.ubuntu.com/ubuntu bionic-security/main Translation-en [97.6 kB]
Get:15 http://us.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages [446 kB]
Err:16 http://ppa.launchpad.net/mc3man/trusty-media/ubuntu bionic Release
  404  Not Found [IP: 91.189.95.83 80]
Get:17 http://us.archive.ubuntu.com/ubuntu bionic-updates/main Translation-en [193 kB]
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/mc3man/trusty-media/ubuntu bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I'm not quite sure what the issue is here. My goal is to just reset the repository list to the factory default for stability and re-add things as they become necessary, but it seems that the default configuration isn't working well.

That's not the default configuration. It seems like you didn't wipe the sources lists in /etc/apt/sources.list.d/.