An old mate made a kit and kilo with a full kg of molasses. He said it tasted "like Molasses".
Ended up diluting it with 4 full batches of normal beer, ending up with 115 litres of Porter.
From Palmer's book his advice on using 'creative' ingredients is to decide how much to add, and then HALVE IT!
I have also read somewhere to only use Blackstrap Molasses. I get mine from the sugars aisle in the supermarket,
usually $3-4 for a 600 g jar.
Look on the label for the sugar content, usually around 65%. This means that the other 35% are unfermantable
and will remain to add flavour, after all that's why you are adding it.
You can always make a second batch with more of your special ingredient if a it's too subtle flavour.
I have added 0.3 kg of Molasses to brown porter with excellent results. I would think the 0.5 kg is the above recipe from Forever Wort is the absolute maximum I would recommend.
I would definitely not be making a beer using 3 kg of molasses as the base sugar, it won't be beer, it will be fermented tar.