In light of Biblical cosmology, it would be out of character for the author to speak of a particular heaven and not also have in mind a corresponding location with respect to the earth, and the corresponding heavens.
Visually the earth is covered by a hard beaten out structure called the firmament, then above it is another heaven. A physical place but with spiritual (supernatural) properties. The first heaven is up to the clouds and the second heaven is from the clouds to the third heaven, it includes the firmament.
What takes place in the second heaven is pertinent to the question of the singular heaven. This is informed by the OT Scriptures. This is the location of the divine council. It is in “heaven” as per
“For who in the skies (בשׁחק or νεφελαις) can be compared to the Lord? Who among the heavenly beings is like the Lord, a God greatly to be feared in the council of the holy ones, and awesome above all who are around him?”
More specifically above the clouds. Hence the understanding and title of cloud walker/rider
““I saw in the night visions, and behold, with the clouds of heaven there came one like a son of man, and he came to the Ancient of Days and was presented before him.”
The Most High resides above the clouds. This is where He sits on the throne and pronounced judgments. It is a desirable place, a place the Accuser wanted to attain for himself.
“I will ascend above the heights of the clouds; I will make myself like the Most High.’”
He did not say above the firmament, all activity happens below the firmament. Nothing crosses the firmament. Only God is above the firmament. The divine council is not merely where the Most High or Elohim resides, but the sons of God with whom He consults also reside here and later in earth’s history, unclean spirits (gods/principalities) would also attend, 1 Kings 22.
“God has taken his place in the divine council; in the midst of the gods he holds judgment:”
Why is only one heaven referred to in the Lord’s Prayer and specifically which of the three?
The answer is clear. The place from where judgments are made. The divine council. The divine council convocations are above the clouds, where the sons of God meet. The very portion of the Lord’s Prayer indicates that decisions are being made and God’s will, decision, judgments are followed through, from heaven onto earth.
“Your kingdom come, your will be done, on earth as it is in heaven (singular).”
It is also the same picture that Paul paints. Our battles and wresting is not on earth and not with flesh and blood but in a heavenly place from where we are seated in the spirit with Christ (thus the Christian is both present in the physical body on earth and spiritually in the spirit, IN CHRIST seated in heaven).
“For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places (different regions in heaven corresponding directly above the geographic places on earth).”
“and raised us up with him and seated us with him in the heavenly places (as Christ has dominion over all of the second heaven in all regions not just directly above the land mass of Israel) in Christ Jesus,”
Jesus is said to have come down from the third heaven and this is where He has seen the Father
“No one has ascended into heaven except he who descended from heaven, the Son of Man.”
The word for no one ουδεις would indicate that not even the angels have been to that third heaven, no one and nothing except the Son of Man. It says that only Jesus has been in the third heaven and this according to other Scriptures occurred not prior to incarnation but prior to the Creation of the heavens and the earth.
To answer the last question why then does it say heavens about the Father? Because God the Father being a Spirit without a heavenly or earthly body is not hindered by the limitations of a body and is able to be present everywhere, hence His omnipresent nature.
Jesus tells the disciples he is going to prepare a place
“In my Father’s house are many rooms. If it were not so, would I have told you that I go to prepare a place for you?”
The word here for “house” is οικια which could really be translated as the place of residence. In other words Jesus is going to the third heaven to prepare this place. That’s why John had to enter through the firmament to go into the third heaven
“After this I looked, and behold, a door standing open in heaven! And the first voice, which I had heard speaking to me like a trumpet, said, “Come up here, and I will show you what must take place after this.””
That place cannot be described.