Heaven opened by the practice of the three Hail Marys

Catholictradition.org

One of the greatest means of salvation, and one of the surest signs of predestination, is unquestionably, the devotion to the Most Blessed Virgin Mary. All the holy doctors of the Church are unanimous in saying with St. Alphonsus Liguori: "A devout servant of Mary shall never perish."
The chief thing is to persevere faithfully until death in this devotion.
Can there be an easier or more adaptable practice for all than the recitation each day of three Ave Marias in honor of the privileges conferred by the Adorable Trinity on the Blessed Virgin?
One of the first to say the three Hail Marys and to recommend them to others was the illustrious St. Anthony of Padua. His special aim in this practice was to honor the spotless Virginity of Mary and to preserve a perfect purity of mind, heart and body in the midst of the dangers of the world. Many, like him, have felt its salutary effects.
Later on, St. Leonard of Port-Maurice, the celebrated missionary, had the three Ave Marias recited morning and evening in honor of Mary Immaculate, to obtain the grace of avoiding all mortal sins during the day or and night; moreover, he promised in a special manner eternal salvation to all those who proved constantly faithful to this practice.
After the example of these two great Franciscan Saints, St. Alphonsus Liguori adopted this pious practice and gave it his most ardent and powerful support. He counseled its use and even imposed it as a penance on those who had not adopted this good habit. The holy Doctor exhorts, in particular, parents, and confessors to watch carefully that children be faithful in reciting each day their three Hail Marys, morning and evening and recommended it to  all the devout young or old.
It is this Saint who suggested adding the aspiration after each Hail Mary: "By thy Immaculate Conception, O Mary, make my body pure and my soul holy."

This practice has been revealed to St. Melchtilde with the promise of a good death, if she was faithful to it every day. 

It is also written in St. Gertrude's revelations:
"While this Saint sang the Hail Mary, at the matins of the Annunciation, she suddenly saw spring out from the Heart of the Father and of the Son, and of the Holy Ghost, three bright flames which penetrated the Heart of the Holy Virgin." Then she heard the following words:
"After the Power of the Father, the Wisdom of the Son, and the
merciful Tenderness of the Holy Spirit, nothing approaches the
Power, the Wisdom and the merciful Tenderness of Mary."
His Holiness, Benedict XV raised the Confraternity of the Three Hail Marys to an Archconfraternity and accorded it indulgences.
Our Lady requested the daily recitation of three Hail Marys, revealing the following to St. Melchtilde:
"The first Hail Mary will be in honor of God the Father, Whose omnipotence raised my soul so high above every other creature that, after God, I have the greatest power in Heaven and on earth. In the hour of your death I will use that power of God the Father to keep any hostile power from you.
"The second Hail Mary will be in honor of God the Son, Who communicated His inscrutable wisdom to me . . . In the hour of your death I will fill your soul with the light of that wisdom so that all the darkness of ignorance and error will be dispelled.
"The third Hail Mary will be in honor of God the Holy Ghost, Who filled my soul with the sweetness of His love and tenderness and mercy . . . In your last hour I will then change the bitterness of death into Divine sweetness and delight."
PROMISE:
During an apparition to St. Gertrude, the Blessed Mother promised, "To any soul who faithfully prays the Three Hail Marys I will appear at the hour of death in a splendor of beauty so extraordinary that it will fill the soul with Heavenly consolation."
 
ACT OF CONSECRATION TO THE BLESSED TRINITY 

With all my heart I praise Thee, Most Holy Virgin
above all Angels and Saints in Paradise, Daughter of
the Eternal Father, and I consecrate to Thee
my soul with all its faculties.
THE HAIL MARY
Hail Mary, full of grace, the Lord is with Thee!
Blessed art Thou among women and blessed is the fruit of
Thy womb, Jesus. Holy Mary, Mother of God,
pray for us sinners now and at the hour of our death. Amen.
With all my heart I praise Thee, Most Holy Virgin
above all Angels and Saints in Paradise, beloved Mother
of the Son of God, and I consecrate to Thee my body
with all its senses.
Hail Mary, etc.
With all my heart I praise Thee, Most Holy Virgin
above all Angels and Saints in Paradise, beloved Spouse
of the Holy Ghost, and I consecrate to Thee my heart
with all its affections, and beseech Thee to obtain for me from
the Most Holy Trinity all the graces necessary for salvation.
Hail Mary, etc.
PRACTICE: Recite morning and evening the Consecration and Three Hail Marys in honor of the three great privileges of Mary,
together with this invocation at the end of each Hail Mary:
By thy holy and Immaculate Conception, O Mary,
make my body pure and my soul holy;

preserve me this day [this night] from mortal sin. 

Imprimatur: Feb. 7, 1963

Francis Cardinal Spellman, Archbishop of New York

Pope St. Pius X gave his Apostolic Blessing to this practice.

The devotion was raised to an Archconfraternity by Pope Benedict XV.

This devotion in its present form is from St. Leonard of Port Maurice.