Hello,
I've setup up 3 external apps for 3 subdomains and i've used the following rewrite rule: (on example.com)
RewriteCond %{HTTPS} on
RewriteRule ^(.*)$ http://subdomain1_ext_proxy/$1 [P,E=Proxy-Host:subdomain1.mydomain.com]
RewriteRule ^(.*)$ http://subdomain2_ext_proxy/$1...