Mat 5:3 Blessed are the poor in spirit: for theirs is the kingdom of heaven.
Wealthy [are] the beggars who have breath because they are part of the universal realm.
This verse is a lot more complicated than the KJV, which only approximates it.
The word "blessed" in Greek is an adjective from a root word meaning "happy" or "fortunate" but with the sense of favored by God. In Christ's era, all luck was attributed to divine favor. It can also mean "wealthy". It it is the adjective "blessed", this sets up an interesting play on words that only works in Greek, saying "wealthy the poor,".
There is not "are" here. These were spoken statements, not written sentences.
The Greek word translated as "poor" means "a beggar." It is the subject of the phrase and plural, "beggars." The first two words are literally, "Lucky beggars."
The most interesting aspect of the next word, translated as "of spirit". For more about its meaning, read this article on soul, spirit, mind, and heart and this article on "holy" "spirit." Though this word is translated as "of spirit" it is not in the normal possessive form. It is in the form of a direct object which can use used for a number of things in Greek including the possessive, but that would be unusual for Christ. It could indicate spirit as an instrument: "lucky beggars, using spirit." It could indicate spirit as a benefit, "lucky beggars for their spirit." It could also indicate a location, "lucky beggar in spirit."
Given the meaning of the words, there are many potential interpretations, all legitimate. Christ could be saying that the poor are wealthy in the spirit of God. It could mean that the poor are lucky because they are in a spiritual place. He could be saying that they are fortunate because they can use spirit as in instrument. Or it could mean simply that even the poorest of us are lucky to be alive.
The KJV uses "in" which should indicate a location in time or place. If we mean beggars who have spirit, a better way to say this is "beggars who have spirit" making it the possessive dative.
The "for" here is a causal adverb that means "seeing that", "because", or "since."
This brings us to the "theirs." this is the possessive plural pronoun. It refers to the noun "kingdom".
Again, the possessive has many uses in Greek . It could describe a producer, "they produce the kingdom"). It could describe an object's nature, "the kingdom consists of them." It could describe an attribute, "the kingdom is like them." It could describe ownership, "the kingdom is owned by them." It could also mean that the poor as a part of the kingdom, one of its groups of citizens.
This brings us to understanding what the "kingdom of heaven" is. Literally, this means the "realm of the skies." This article on The Kingdom of Heaven discusses it further and it is updated regularly.
A contrasting of adjectives meaning "wealthy" and "poor"
The Spoken Version:
“Lucky! The beggars!” He said in a cheerful tenor, indicating a group of beggars who were seated near the stage area.
Many laughed at the idea of beggars being lucky.
“For the breath of life,” he continued earnestly, closing his eyes and taking a deep breath. He spoke the common tongue like a local, but with a slight accent.
“Because theirs is,—the realm of the skies!” he said happily.
τῷ πνεύματι (noun sg neut dat) "Spirit" is pneuma (pneuma), which means "blast", "wind", "breath", "the breath of life", "divine inspiration", "a spiritual or immaterial being," and "the spirit" of a man.
αὐτῶν (adj pl masc gen) "Theirs" is from autos (autos), which means "the same," and the reflexive pronouns, "myself", "yourself", "himself", "herself", "itself," or the oblique case of the pronouns, "him", "her," and "it." It also means "one's true self," that is, "the soul" as opposed to the body and "of one's own accord."
τῶν οὐρανῶν. (noun pl masc gen) "Heaven" is from the Greek ouranos (ouranos), which means "heaven as in the vault of the sky", "heaven as the seat of the gods", "the sky", "the universe," and "the climate."