2 If indeed. The author is not doubting whether his audience has heard, but is rather using provocative language (if indeed) to engage his audience in thinking about the magnificence of God’s grace. However, in English translation, the apodosis (“then“-clause) does not come until v. 13, leaving the protasis (“if“-clause) dangling. Eph 3:2–7 constitute one sentence in Greek.
3 Or “administration,” “dispensation,” “commission.”

In other versions

assuming that you have heard of the stewardship of God’s grace that was given to me for you,

Surely you have heard about the administration of God’s grace that was given to me for you,

assuming you have heard about the administration of God’s grace that he gave me for you.

assuming, by the way, that you know God gave me the special responsibility of extending his grace to you Gentiles.

—if indeed you have heard about the stewardship of God’s grace given to me for you.

If ye have heard of the dispensation of the grace of God which is given me to you-ward:

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