"worked on", "man" and" hour"

[Egyption Arrow]

In the name of the Merciful Allah,
Hi, in a study on "prison life", scientists built a mock prison and asked participants to live in, acting the roles of guards and prisoners. The prison had three small cells with three prisoners, another three were assigned to each cell. Here is what I read and need help about: " The guards worked on three_man eight_hour shifts, and went home after their shifts". I know that "work on somebody" means trying to persuade or influence him, but it doesn't fit with my quote. About" three_man", why "man" is not in plural? Likewise, why hour is not hours in " eight_hour shifts", since the number of hours is not one? Thanks alot ahead.

[engee30]

In the name of the Merciful Allah,
Hi, in a study on "prison life", scientists built a mock prison and asked participants to live in, acting the roles of guards and prisoners. The prison had three small cells with three prisoners, another three were assigned to each cell. Here is what I read and need help about: " The guards worked on three_man eight_hour shifts, and went home after their shifts". I know that "work on somebody" means trying to persuade or influence him, but it doesn't fit with my quote. About" three_man", why "man" is not in plural? Likewise, why hour is not hours in " eight_hour shifts", since the number of hours is not one? Thanks alot ahead.

worked on ... refers to the noun ... shifts which is modified by the attributive three-man eight-hour. I think that there were three guards on each eight-hour shift.
When numbers are used in an attributive, the nouns connected with them are normally in a singular form:

a three-year-old boy
a three-man crew
a three-hour shift

[banderas]

5 star hotel not 5 stars hotel