While Devtoolset-3 and 4 are located on the Centos mirror site, yum will not be able to find them as they were not included in the repodata on the site. These were probably excluded because they are EOL (end-of-life), and have dependency bugs (Bug 1410152). Try using the older Devtoolset-3 packages here, which do not have the dependency issues:

https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/

If you really want the packages from the centos.org site, I went with wget to download all the RPMs, then recreated a local yum repo. However, you'll see the dependency issues and yum install devtoolset-3 will fail.

$ wget -r -p -e robots=off --directory-prefix="/mnt/local-devtoolset3-repo" --recursive --no-clobber --no-parent http://mirror.centos.org/centos/6/sclo/x86_64/rh/devtoolset-3/
$ sudo yum install createrepo
$ cd /mnt/local-devtoolset3-repo
$ createrepo --database .

I am using CentOS 7.9 and I encountered the same problem after following instructions here to install and run gcc 11. I tried launching different versions of gcc and found only devtoolset-9 works, which corresponds to the file devtoolset-9 in /etc/scl/conf/ folder. So I copied devtoolset-9 to devtoolset-11 in the same folder, and gcc 11 gets working.