Currently, running Tumbleweed + GNOME3. I noticed that GNOME3 Software doesn’t show some major applications in their respective categories. For instance, Blender is not listed under Graphics/3D Graphics, until it is installed via zypper or YaST2 beforehand.
Where does Software get its information from? Is it possible to somehow refresh/sync the list of available software with the repos so that Software shows all major apps? 
Well, if they’re not installed, they’re not going to show up. Unless you are talking about showing up in the software installer module in YAST.
And if it’s the latter, then maybe you haven’t added any other repos (such as pacman).
Maybe if you give us your repo list:
zypper lr -d
Here is the output of zypper lr -d:
# | Alias | Name | Enabled | GPG Check | Refresh | Priority | Type | URI | Service
--+----------------------------------+---------------------------+---------+-----------+---------+----------+--------+---------------------------------------------------------------------+--------
1 | download.opensuse.org-non-oss | Main Repository (NON-OSS) | Yes | (r ) Yes | Yes | 99 | yast2 | http://download.opensuse.org/tumbleweed/repo/non-oss/ |
2 | download.opensuse.org-oss | Main Repository (Sources) | Yes | (r ) Yes | Yes | 99 | yast2 | http://download.opensuse.org/source/tumbleweed/repo/oss/ |
3 | download.opensuse.org-oss_1 | Main Repository (OSS) | Yes | (r ) Yes | Yes | 99 | yast2 | http://download.opensuse.org/tumbleweed/repo/oss/ |
4 | download.opensuse.org-tumbleweed | Main Update Repository | Yes | (r ) Yes | Yes | 99 | rpm-md | http://download.opensuse.org/update/tumbleweed/ |
5 | openSUSE-20160228-0 | openSUSE-20160228-0 | Yes | (r ) Yes | Yes | 99 | yast2 | http://download.opensuse.org/tumbleweed/repo/oss/ |
6 | packman | packman | Yes | (r ) Yes | Yes | 99 | rpm-md | http://ftp.gwdg.de/pub/linux/misc/packman/suse/openSUSE_Tumbleweed/ |
7 | repo-debug | openSUSE-Tumbleweed-Debug | No | ---- | Yes | 99 | NONE | http://download.opensuse.org/debug/tumbleweed/repo/oss/ |
I do not mean the Software Management facility within YaST2, but the GNOME3 Software application. It’s present on any GNU/Linux distro using GNOME3 as its desktop environment. On Fedora it properly lists all applications available from the repos. On openSUSE Leap 42.1 and Tumbleweed it only shows some of them. Apps like Blender (3D graphics software) or Spyder3 (Scientific PYthon Development EnviRonment) can be searched for and installed through either YaST2 or directly zypper, but neither pops up in the Software app.
Why is this the case?