Bomb disposal experts, police and firefighters are assessing a house where "substances" have been found, leading to a number of properties being evacuated.
Humberside Police said a 15-year-old boy, a 49-year-old woman and a 56-year-old man were arrested and are in police custody after officers were called to the address in K ilnsea Grove, Hull, at 2.49am today.
A spokesman said: " A police cordon is currently in place and a small number of properties in the vicinity have been evacuated by the police and Humberside Fire and Rescue as a routine precaution."
The spokesman confirmed the Army's Explosives Ordnance Disposal Unit had been deployed as part of a multi-agency response.
He said they were " investigating a report of substance(s) which are believed to be inside a residential property".
Members of the public have been advised not to go near the area and observe the cordon in place.
Detective Inspector Jason Gourley, of Humberside Police, said: "Our officers arrived at an address in east Hull in the early hours of today.
"Since then, we have arrested three people who are currently assisting us with our enquiries in connection with substances believed to be at a property on Kilnsea Grove.
"We are working closely with our partner agencies to ensure the area is safe for the local community, which is why we have put a cordon in place.
"We hope that residents can return to their properties as quickly as possible, but public safety is paramount, so we hope that everyone will obey the police cordons and we will let them know when the police has finished carrying out all relevant enquiries."
The incident is not being treated as terrorist-related, police said.