ssh configuration file environment variable?

This is a bit misusing of the whole concept and twisting the tools to your specific use case. You should rather use the tools as they should be. The configuration files are by default in ~/.ssh/config.

• Why not to create a symlink from there to your ~/workspace/etc/ssh_config on all three computers to make it work out of the box without all that work around?

• Since OpenSSH 7.3 you can use Include directive to achieve the same without symlinks (so you can have also different configuration on some hosts if you need).

• The same trick can be done with system-wide configuration in /etc/ssh/ssh_config.

As you have already have ... add a reference to my custom bashrc ... you can put together also modification of the .ssh/config together in some script in your workspace.