It happened as the driver travelled along the A57 in Worksop shortly after 9pm last night.
He pulled over to call the police and the other car carried on.
Armed police were dispatched as it was unclear whether the gun was real or not.
The suspect car was located a short time later and its three occupants, aged 24, 26 and 31, were arrested on suspicion of possession of a firearm with intent to cause fear of violence.
The vehicle was searched and a plastic toy handgun was seized by officers.
Inspector Chris Pearson, of Nottinghamshire Police, said: “This would have been a very frightening experience for the victim.
“Although it turned out to be a toy handgun, it would certainly have looked like a real one. Imitation firearms can be indistinguishable to live firearms and we will always take them very seriously when we find them.
“Officers did a tremendous job in locating the suspect vehicle so quickly and making three arrests.
"The victim has been left shaken but thankfully was unharmed and, although incidents like this are incredibly rare, we will always treat them with the utmost severity.
"Our message is simple: if you mess around with guns - real or not - we are going to arrest you.
"Reducing the availability of firearms and taking guns off the streets is a priority for Nottinghamshire Police.”