One thing that CakePHP 3 documentation has you do when you install is put the CakePHP Core inside the vendor directory of the app. If you want to share the CakePHP core between multiple apps you can follow these simple steps.
Step 1: Install the CakePHP 3 App
cd {webroot}
composer create-project -s dev cakephp/app ./
In this example, I am going to put it inside `/CakeCore/cakephp` but you can put it wherever.
mkdir /CakeCore
mv {webroot}/vendor/cakephp/cakephp /CakeCore/cakephp
Lastly we need to point the app to the newly cloned core.
cd {webroot}/App/Config
vim paths.php
define('CAKE_CORE_INCLUDE_PATH', ROOT . '/vendor/cakephp/cakephp');
define('CAKE_CORE_INCLUDE_PATH', '/CakeCore/cakephp/');