You need to go into the code of the home page (HomePage.php), create it if it doesn't exist in mysite/code and then add three more database fields like so:
static $db = array(
'Content1' => 'HTMLText',
'Content2' => 'HTMLText',
'Content3' => 'HTMLText'
);
Then add the code for the fields into the CMS below (in the same file):
function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab("Root.Content.Main", new HtmlEditorField("Content1", "Content1 Title"));
$fields->addFieldToTab("Root.Content.Main", new HtmlEditorField("Content2", "Content2 Title"));
$fields->addFieldToTab("Root.Content.Main", new HtmlEditorField("Content3", "Content3 Title"));
return $fields;
}
Then finally add the values in the template HomePage.ss (create the page is it doesn't exist... make a copy of Page.ss in themes/(theme)/templates/Layout, rename to HomePage.ss and then add the fields you created ($Content1 $Content2 $Content3).
See http://doc.silverstripe.org/doku.php?id=tutorial:2-extending-a-basic-site for more info about creating new page templates.