Write .htaccess for siteground php hosting with multiple domains in one website

I have a website created in siteground with domain A.
I will now park domain B and domain C into this website.
And I want to set up different folders for different php websites as following:
public_html/domainA/A -> domainA.com/app/*.php
public_html/domainA/B -> domainB.com/app/*.php
public_html/domainA/C -> domainC.com/app/*.php

Please help write a .htaccess to support the url rewrite (not redirection)