At present time Devtoolset-5 (and earlier) cannot be istalled from Software Collection Repository (centos-release-scl) using yum (only Devtoolset-6 and 7 are available). Nevertheless, this collection can be downloaded as a list of rpm-packages. Is it possible to enable such EOL collections for yum, or maybe there is another way to install them correctly?
You can install the obsolete devtoolset-3 in this way:
sudo yum --obsolete install devtoolset-3