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From what I can tell on p106 of the IK codex, it seems like these following examples are correct:

1) A Super-Heavy Detachment comprising one Crusader and two units of Armigers would get +0CP.
2) A Super-Heavy Detachment comprising two Crusaders and one Castellan would get +3CP.
3) A Super-Heavy Detachment comprising two Crusaders, one Castellan, and one Armiger would get +3CP.

However, the recent errata for p89 of the IK codex made a correction. It lists two Super-Heavy Detachments, one of which has three TITANIC units and gets +6CP, the other of which has less than three TITANIC units and gets +3CP.

I can't see what I'm missing

It's further down the errata. In your example, 1 & 2 would both be +3CP, and 3 would be +6CP.

Page 106 – Knight Lances ability
Change the last sentence to read:
‘The Command Benefit of each Imperial Knights Superheavy
Detachment is changed to ‘None’ if it does not
contain at least one Imperial Knights Titanic unit,
and is changed to ‘+6 Command Points’ if it contains at
least three Imperial Knights Titanic units.’

https://whc-cdn.games-workshop.com/wp-content/uploads/2018/06/warhammer_40000_imperial_knights_en.pdf

Reading fail Maybe I should finish a document before asking questions about it. Thanks!