Commentaries:
This is the first mention of eating unleavened bread in context with the events of the day. It is not the first time that unleavened bread is mentioned in terms of the Feast of Unleavened Bread, as that appears in Exodus 12. Here, Moses is inspired to write down that we are to eat unleavened bread because of what the Lord did.
John W. Ritenbaugh
Unleavened Bread and Pentecost
The last phrase—"there shall no leavened bread be eaten"—is tied to what the LORD did so that we understand why we are to do so.
John W. Ritenbaugh
Sanctification and Holiness (Part 1)
Other Forerunner Commentary entries containing Exodus 13:3:
Leviticus 23:6-8
Joshua 5:2-12