Need Three joomla websites to share one database/login (JFusion)?

I have three quickstart template plaza templates that I want to use, but I want to have only one login/database. I know there is a joomla extension called "Jfusion", but I don't know how to use it. You could use one as the main site and have the other as subsites off of the first or have three seperate domains with external links off of the main menu whatever is easier. I will give the cpanel info and have you upload the three sites the way you need to. I don't need anything else done. The design and sample data is all fine. I just need someone to make sure that the database is shared and the login is the same. For someone who knows mysql or the Jfusion this should be fairly straight forward. My budget is $100. Thank you.