A man and two youths threatened victims near to railway stations before taking items including mobile phones.
Macauley Dodd and two teenagers, who cannot be named for legal reasons, were involved in robberies between December 30 last year and January 17 this year.
Wolverhampton Crown Court heard victims were targeted near to Old Hill, Blackheath and Rowley Regis railway stations during the period.
Dodd, 19, of Blewshill Court, Dawley, Telford, pleaded guilty to one count of robbery on December 30 last year.
He and one of the teenagers, a 17-year-old from Smethwick who also admitted the charge, punched and kicked a victim near to Old Hill railway station. They then took his mobile phone.
The victim suffered bruising to his face and head.
Dodd also pleaded guilty to attempted robbery on January 1.
The other teenager, a 16-year-old also from Smethwick, admitted robbery on the same date.
A £10 Nokia mobile phone was taken from the victim.
Dodd also pleaded guilty to charges of robbery and attempted robbery near to Blackheath railway station on January 11.
He also admitted possession of a knife when he was arrested by police on January 13.
The 17-year-old pleaded guilty to a further charge of robbery on January 17 when he took a mobile phone from the victim on a bus on Oldbury. He also admitted breach of a youth rehabilitation order.
The court was told the 16-year-old had also escaped lawful custody after being arrested on January 20.
He pleaded guilty to the charge.
Dodd was sentenced to five-and-a-half years detention and the 17-year-old three years and eight months detention.
The 16-year-old was given a detention and training order for four months.
Mr Nick Wadsworth, prosecuting, said: "These offences all took place around public transport areas.
"The victims say it has affected them and the items stolen are mainly valued between £300 and £400."
The court was told on Friday the two youths both had a difficult upbringing which had affected their decisions.
Mr Richard Gibbs, defending the 16-year-old, said although his client was still young he was starting to see things from an adult viewpoint.
And Mr Anthony Bell, defending Dodd, said his client already had plans to 'make a fresh start' when he left custody.
The court was told the 17-year-old had 'no desire to continue offending'.
Judge John Warner said all of the robberies had been serious and had a serious impact on the victims.