Here "meek" and "lowly in heart" mean basically the same thing. Jesus combines them to emphasize that he will be much kinder than the religious leaders. Alternate translation: "I am gentle and humble" or "I am very gentle" (See: [[rc://en/ta/man/translate/figs-doublet]])
Here "heart" is a metonym for a person's inner being. The phrase "lowly in heart" is an idiom that means "humble." Alternate translation: "humble" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-idiom]])
Here "soul" refers to the entire person. Alternate translation: "you will find rest for yourselves" or "you will be able to rest" (See: [[rc://en/ta/man/translate/figs-synecdoche]])