What are the first three centuries that the Messenger, may Allah bless him and grant him peace, described as righteous, are they the first three hundred years, and when do these centuries end?

Scholars have differed in defining what is meant by "qarn" (century or generation). An-Nawawi, in his commentary on Sahih Muslim, mentioned several opinions, including: that his ﷺ "qarn" refers to his Companions, then those who followed them (their children), and the third are their grandchildren. Another opinion is: whoever an eye saw and remained, then whoever an eye saw who saw him, and so on. Another is: that a "qarn" is every generation that lived concurrently. The soundest opinion is that his ﷺ "qarn" refers to the Companions, the second to the Tabi'un (Successors), and the third to the Tabi' Tabi'in (Successors of the Successors).