A man, a woman and a child have been found in a trailer which had been transported from mainland Europe.
Police were called to water pump company Selwood in Chandler's Ford industrial estate, Hampshire, on Wednesday afternoon after the driver raised the alarm.
Hampshire Constabulary said in total it had arrested two men and a woman on suspicion of immigration offences.
They remain in custody. The Home Office said at least one man was Sudanese.
It added: "His case is being progressed in line with the immigration rules.
"Where someone has no right to remain in the UK, we will take action to remove them."
A statement from Selwood said: "A member of staff at the security gate alerted the police after the driver of a lorry which had stopped before entering the site raised concerns that there may be people in the vehicle.
"The lorry was due to make a delivery to Selwood.
"The matter was handled by the police and the lorry, which had been sent by a supplier from Germany, was taken onto the site at Chandler's Ford."