I would like to map
Shift+Esc to Esc Esc Esc
on my MacBook Pro.
I have tried Better Touch Tool. It can remap one key combination to another, but not to a sequence of keystrokes.
Within what application do you want this binding? I'm guessing Terminal or iTerm.
For iTerm, you can select Preferences > Keys then add a global shortcut to the action you want.
For Terminal it's similar. Preferences > Keyboard but you'll have to know the keystroke sequence in hex (I believe ESC is 0x35)
The other way to set shortcuts is in System Preferences > Keyboard > App Shortcuts. This is the easy way to map a menu option but may not be appropriate for your use case.
For a general solution you'll want to use Karabiner. It'll let you remap things for all your applications and it comes with tons of options as well as the ability to customize and create your own mappings if you want. Check out this answer for an example of how to go about creating new mappings or check out their (fairly friendly) documentation here.