Do I want a multisite?

Hello,

I have added my site onesite.com. I also own 2 other domains (sitetwo.com; and Onesiteblog.com) that are meant to be connected to my main site, as one is a sub-brand and the other is my brands blogsite. I want them to be separate where they have their own logos at the top of the site etc but using the same system. For example Nike.com has Hurley.com and Converse.com connected. But separate. I want customers to be able to sign up for one site and be members for the others as well. I want them to shop at onesie.com and add items from sitetwo.com to the same cart. Is this possible? What is the best way to do this?

Thanks