The word heart is used five times in these three verses to discuss the influence of Satan over the Nephites who had wandered into apostasy and forgot the sign of the Savior’s birth.
In verse 1, they became “hard in their hearts, and blind in their minds.” They were less impressed by and even disbelieving the wonders and miracles they had seen.
In verse 2, they were imagining “some vain things in their hearts” that those miracles had been conjured up by man with the aid of the devil in order to “deceive the hearts of the people.” Thus, Satan controlled their hearts and led them into disbelieving the doctrine of Christ.
In verse 3, as the people began to resist the Gospel and failed to keep the commandments, Satan continued to lead away their hearts, “tempting them and causing them that they should do great wickedness in the land.”
This makes it clear that apostasy is largely a condition of the heart. Once the people forgot, took for granted, and then rationalized the blessings, they stopped believing and were easily led into sinful behavior.