This is an interesting side of the debate on, but I really don't think there actually was much of a motive in this case .. I think Damien at the time was interested in alternative religions, just as he seems to be now.
IMO there was no planning here, whatsoever. The 3 older boys were drinking in the woods and by chance the three younger boys entered the area, and then the attack began, sort of like an act of bullying with Damien and Jason each outdoing the other which tragically leads to the murder of all three.
I think there's a difference between someone in a 'hate gang' for instance going out and seeking a target for their misguided rage, or perhaps setting upon someone they were already felt a racist hatred for, and this crime .. however I understand the parallel of vulnerable victims.
I'm sure there was a feeling of power and dominance gained simply by being able to commit the act, and by doing it in front of others. I also think that's part of the reason Damien talked about it afterwards, it added to his cache for want of a better word, or so he thought.
He may have even tried to frame the act in his own mind to give it deeper meaning for his own benefit, but I really don't believe this crime had much sense to it at all, in the same way I don't believe school shootings make much sense. I view them similarly.
Having said that, I can see Damien adding value and power to the act post crime, especially in discussions with Jason.
MGN...I have to say, even though I don't think there was enough evidence to establish their guilt, if I were asked to describe a scenario in which the WM3 did commit the murders, it would probably closely follow what you have laid out. I think it would have been a crime of opportunity. In other words, it was not planned out beforehand. The WM3 were there, the 3 boys happened to come along and the opportunity arose. Any motivation came from the desire to exercise power over another, not some mystical, hocus pocus belief that power was going to be gained by doing it.