A 21-year-old inmate was killed at Pentonville prison in a bloody battle to control the wing's "lucrative" contraband route, a court heard.
Basana Kimbembi, 35, Joshua Ratner, 27, and Robert Butler, 31, allegedly murdered new father Jamal Mahmoud, at the north London jail on October 18 last year.
Opening their Old Bailey trial, prosecutor Mark Heywood QC described how Mr Mahmoud was attacked with such "calculated brutality" that it shocked even hardened prisoners.
He said: "He was killed with calculated brutality by a group of men armed with weapons that went to find him and those with him.
"That lethal violence was quite deliberate and expected. When it happened, both sides knew that it was going to happen and were prepared to engage in it. In the event, the level of it was shocking, shocking to those who lived in that place.
"The second most shocking aspect of this death is that it took place in one of Her Majesty's prisons, on a wing populated with people who had been there for some time, overseen 24 hours a day by regular staff.
"The sheer determination of the killers is indicated by the fact that neither the location, the security or the presence of prison staff served to stop it or deter the violence or the weapons used to drive it home.
"The Crown's case is that these three defendants together armed themselves in advance and went to confront Jamal Mahmoud and his group who also were armed.
"They did so to make their point and to succeed in resisting the others and getting control of part of the lucrative contraband route onto that wing of the prison.
"In doing so, they acted together, intending to use the weapons to cause really serious, even lethal harm: to confront."
The defendants have denied murder as well as wounding a second man, Mohammed Ali, with intent to cause him grievous bodily harm.
By mid-October last year, a "serious dispute" had developed, the court heard.
One of the defendants was to later tell prison authorities that Mr Mahmood was "playing with the big boys".
The day before the killing, a package had come into the prison and there was open talk of a "beef", jurors heard.
A civilian employee saw a length of sheet dropped from the fifth floor window over the prison wall into the street before it was pulled back, jurors heard.
It was clear weapons were on the wing, jurors were told. The victim was seen by another inmate to visit Butler and Kimbembi in a cell.
During their discussion, Kimbembi allegedly pulled a large combat knife from a sheath and told him to move back.
Mr Mahmood left, saying "We'll see what happens" and Kimbembi allegedly looked up and said "I am going to kill you".
Mr Heywood said: "Whatever he meant by those words, the fact is he did. He did so within about 24 hours."