Successfully installed Craft 3 via Composer but can't access site or admin
On the public_html/index.php file, try replacing 

define('CRAFT_BASE_PATH', dirname(__DIR__));


with

define('CRAFT_BASE_PATH', ('/home/pathToCraftFolder/craft'));


The structure of my folders is as follow:

/home/username/
  - public_html, or just html / this is where the content of your web folder goes  
  - config
  - modules
  - storage
  - templates
  - vendor
  - (and then the composer/env files)

Then on the index.php file (which can be found under /home/username/public_html/index.php),you'd replace:

 define('CRAFT_BASE_PATH', dirname(__DIR__));


with

 define('CRAFT_BASE_PATH', ('/home/pathToCraftFolder/'));


If you only have access to the public_html folder or would like to install craft3 on a subfolder this is how you can structure it:

on your public_html (/home/username/public_html),
create a craft subfolder((/home/username/public_html/craft)), then send your craft3 web folder into the public_html. It should look like this:

/home/username/public_html/
               -index.php
               -web.config
               -.htaccess
               /craft/

In your index.php file, replace

define('CRAFT_BASE_PATH', dirname(__DIR__));


with

 define('CRAFT_BASE_PATH', ('/home/pathToCraftFolder/craft'));


Hope this helps.