The first took place at about 6.30pm on Tuesday when a 20-year-old man was approached by two men outside a Nisa shop in North Road, Bellshill, North Lanarkshire.
He was threatened with a knife and forced to hand over his mobile phone. The thieves then made off on foot towards Hilltop Avenue.
At about 8.30pm, a 35-year-old man was robbed of his mobile phone by two men with a knife as he waited on a bus in Main Street, Bellshill. The robbers made off towards Muirmadkin Road.
Then, at about 9.10pm an 18-year-old man had his phone stolen in Carfin Road, Motherwell, after leaving a Nisa store. The two robbers left in the direction of Whittagreen Avenue.
The robbers are aged between 16 and 20, about 5ft 8in and slim. Both wore dark-coloured hooded tracksuits, one of which had white stripes. One had a scarf covering the lower part of his face and both had their hoods up.
Detective Sergeant Craig Hosie, of Bellshill police office, said: "This was a very frightening experience for these men who thankfully were not injured.
"We need the public's help to catch those responsible as soon as possible and I would urge anyone who was in any of these areas and saw someone fitting the descriptions or acting suspiciously to contact the police.The incidents may be connected."
Officers from Strathclyde Police can be called on 01698 202 400 or anonymously via Crimestoppers on 0800 555 111.