Musketeer's call for a non-share policy? (3,3,3)

I believe the answer is:

all for one

I'm a little stuck... Click here to teach me more about this clue!

'non-share policy?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'musketeer's call for a' is the wordplay.
'musketeer's call' becomes 'all' (I can't explain this - if you can you should believe this answer much more).
'a' becomes 'one'.
'all'+'for'+'one'='ALL FOR ONE'

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app