I couldn't find an answer to this in these posts, so i thought to post it myself.
Asked, and answered, many times before.
Either
redirect onemainsite.com to onemainsite.com/one via your control panel, or in
/.htaccess:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?onemainsite\.com [NC]
RewriteCond %{REQUEST_URI} !^/one [NC]
RewriteRule ^(.*)$ /one/$1 [R=302]
This assumes that you don't want visitors (and search engines) to see they've been redirected to /one. If you
do, change R=302 to R=301.
If it should end up also redirecting your two add-on domains to /one, it means that they are coming in as onemainsite.com/addonone/ or addontwo/. In that case, add
RewriteCond %{REQUEST_URI} !^/addonone [NC]
RewriteCond %{REQUEST_URI} !^/addontwo [NC]
just before the "RewriteRule" line.
