The text reads "Benefit: Your Dark One’s Blessing now gives you 3 additional temporary hit points." It does not increase the value of your Dark One's Blessing by 3. It grants you 3 additional, as in, in addition to. So your power grants you 7 temporary hit points, in addition, it grants you 3 temporary hit points.
The same text allows for powers (I'm not sure if there actually are any, but there could be in future releases) that read like "gain X temporary hit points for every Y".
The same text allows for powers (I'm not sure if there actually are any, but there could be in future releases) that read like "gain X temporary hit points for every Y".