Maybe I wasn't clear in my description, but I'll try again:
I have webshops on hosting account #1, #2 and #3 (3 different hosting accounts and 3 different domains)
I want to have a blog on a subdomain to domainA.com (like this: blog.domainA.com) where the blog is hosted on another server than the webshop.
I will have total 6 different hosting accounts, 3 for webshops and 3 for blogs. How can I install a blog on a hosting account and "merge" it with a subdomain to a webshop hosted on another hosting account?