Anderton: You are ignoring one of the fundamental guidelines for seemingly contradictory rules.
That guideline is that a more specific rule overrides a more general rule. If you disagree with this guideline then you are in the wrong forum.
The application of this guideline to this situation is clear:
The *3 allows Grey Knights to take Psycannons
but the Wargear specifically says that Grey Knights in Power Armour cannot.
Nobody here is arguing that the Justicar is not a Grey Knight and therefore not affected by the *3 rule.
They are arguing that the more specific rule overrides the more general rule.
Anderton they are right and you are wrong.
Please reference the following:
http://www.dakkadakka.com/Default.aspx?tabid=93&forumid=15&postid=6158&view=topic Specifically note A-2