.htaccess for site

I have three registered domain names (WP Sites) that have somehow been made subdomains of one registered domain name. Fatcow’s tech told me I could fix this with a redirect using .htaccess. They won’t fix this for me. I have no clue what to do. I want to be able to type in each domain name without having to include the /name, for example I have to type http://www.mysitename.com/blog. How do I ensure that all the files associated with my WP site get transferred correctly? I am not a programmer, but can follow directions. Any help is deeply appreciated. Thanks.