Exact location of CI files

Hi John, yes, I would love to see a screenshot of the setup if it's not too much trouble!

I have three domains right now with three separate ci installs. I was wondering if I could somehow just install ci once, but ....aren't the autoload, route, config files all different?

Here's my setup:

PHP Code:

var/www/ci2/
    /Codegniter-3.1.0/system/
    /Codegniter-3.1.1/system/
    /Codegniter-3.1.2/system/                
    /Codegniter-3.1.3/system/
    /Codegniter-3.1.4/system/

var/www/
    html/public_tml/
        index.php        
    site-001.com/public_html/
        index.php
        application-001/        
        application-002/        
        application-003/        
    site-002.com/public_html/index.php        
    site-003.com/public_html/index.php        
    site-004.com/public_html/index.php        
    site-005.com/public_html/index.php        
    john_betong.tk/public_html/index.php        

    // common index.php
        index.php -> 
        # $system_path = 'system';
         # $system_path = '../../ci2/CodeIgniter_2_1_2/system';
          # $system_path = '../../ci2/CodeIgniter_2_1_4/system';
          ...
          # $system_path = '../../ci2/CodeIgniter-3-1-3/system'; 
          $system_path = '../../ci2/CodeIgniter-3-1-4/system'; // 2017-03-20

         # $application_folder = 'application-001';
         # $application_folder = 'application-002';
           $application_folder = 'application-003';