Verse 11, and most of verse 12, are unchanged from Matthew 5:11–12. The change at the beginning of verse 12 does not add to or alter the meaning from Matthew.
Matthew: “Rejoice, and be exceeding glad.”
3 Nephi: “For ye shall have great joy and be exceedingly glad.”
These two verses are a continuation of the saying in verse 10. It repeats the basic theme, and expands upon it. This duplication and expansion are indications that, in the Old World, this was a condition that would come to pass for most who listened. For the New World, it was again more relevant to their immediate past experience. The current Nephites would usher in a period of peace, where this kind of persecution would not be seen again for two hundred years.