Why is blasphemy against the Son or Father forgiven but blasphemy against the Spirit isn't forgiven? Matthew 12:31–32 (RSV)
  31  Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. 32  And whoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.


If the Trinity is real then how come whoever speaks against the Holy Spirit will not be forgiven but whoever speaks against the Son of Man will be forgiven? Aren't the Son, Father and Spirit equal to God? So is speaking against one speaking against all three?
Fr. Cornelius à Lapide commentates on this verse:

Lastly, theologians—and from them, catechists—out of various expositions of S. Augustine, collect six sins against the Holy Ghost; namely, presumption, despair, striving against known truth, envy of fraternal charity, impenitence, and obstinacy. They say that these are called sins against the Holy Ghost, because they are committed through undoubted wickedness against the goodness of God, which is an attribute of the Holy Ghost. Thus, likewise, sins which are committed through infirmity are said to be done against God the Father, because power is one of His especial attributes. And sins which are done through ignorance, are said to be done against the Son, because of His attribute of wisdom.
Note, therefore, that Christ is here speaking not of every sin against the Holy Ghost, but only of blasphemy against the Holy Ghost, which may take place by words; and the same reasoning will apply to thoughts and actions, as when anyone reviles works manifestly divine and miraculous, which God works for the salvation of men, by which He confirms faith and truth. Such a work is the casting out of devils; and because such works proceed from the goodness and holiness of God, they are attributed to the Holy Ghost, who proceeds from the Father and the Son by procession and inspiration, as Love, Goodness, and Holiness. When, therefore, anyone calumniates such things, and knowingly out of malice ascribes them to an unclean spirit (as these Pharisees did), such an one is said to commit blasphemy against the Holy Ghost; for such an one directly fights against God and takes from Him His holiness and purity. The whole argument is expressed in the following syllogism:—
The author of the miracles which Christ performs is, according to you, O ye Scribes, Beelzebub:
But God the Holy Ghost is, in truth, the Author of these miracles:
Therefore, according to you, God and the Holy Ghost are Beelzebub.
What more horrible can possibly be said? What greater blasphemy can be imagined? S. Basil adds that there are such persons even now, who ascribe the fruits and actions of the Holy Ghost to the opposing unclean spirit. We many of us do this, when we call earnestness ambition, and impute the calumny of anger to one who is only moved by zeal and righteous indignation. Moreover, Christ opposes this blasphemy against God and the Holy Ghost to that blasphemy against the Son of Man by which some who were offended at Christ’s human conversation, calumniated what He did as man, as when they called Him a wine bibber, and a friend of Publicans and sinners. This was something more excusable, and less unworthy of forgiveness, because it had respect to Christ as Man rather than as God.
Shall not be forgiven: Arab. Shall not be relaxed, i.e., shall with difficulty, and seldom be forgiven. For this blasphemy against the Holy Ghost is most horrible, inexcusable, and altogether unworthy of pardon, and, considered simply in itself, takes away and excludes all medicine, and means of obtaining forgiveness. For such a blasphemer places himself in diametrical opposition to the Holy Ghost, and drives Him from him, yea blasphemes Him: the Holy Ghost, I say, by whom alone he could be absolved, healed, and sanctified. Similarly, we call an incurable disease one, which does not admit of medicine, and rejects every kind of food. Nevertheless a blasphemer does not shut up the hand of God, so that God cannot have mercy upon him, although unworthy; and convert him, as He converted S. Paul, who confesses that he had been a blasphemer against God (1 Tim. 1:3).