Try:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^/(.*) http://www.%{HTTP_HOST}/$1 [L,R=301]
Try:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^/(.*) http://www.%{HTTP_HOST}/$1 [L,R=301]