Keep this one rule for all the sub-domains:
# rule for forcing www on main domain
RewriteCond %{HTTP_HOST} ^mydomain\.com$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
# rule for removing www on sub domains
RewriteCond %{HTTP_HOST} ^www\.([^.]+\.mydomain\.com)$ [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [R=301,L]