Two of the men died instantly and the third died of his injuries later after they were caught by a sudden rush of gas at the Howdon yard of Amec, one of the North-east's leading offshore companies. Four other men were hurt in the accident, which happened late on Thursday on a gas rig as nitrogen was being used to test piping systems.
The men who died were all close to the point of release and took the full force of the blast.
One of the injured men said: "As far as I know they were taking a flange off when it happened. I was just assisting the riggers when it went up. The next thing I knew I was just wandering around."
About 40 firefighters were called to the yard but there was no blaze and little immediate sign of damage. Station Officer George Arnott said: "It was a strange scene - nothing seemed out of place except there were three casualties lying there seriously injured. Once we started inspecting the site, however, we realised something quite severe had happened."
The incident was a release of gas under pressure rather than an explosion, he said, like bursting a balloon under very high pressure. "It has probably thrown the men back quite forcibly and at quite a speed." The dead and injured workers were understood to be members of the GMB union, which represents most of the labour force at Amec.
Martin Gannon, a regional union official, said: "Our hearts go out to the families of the victims . . . A full inquiry will be held by the Health and Safety Executive and we will be asking for all the details and will be fully involved in that inquiry. We will be looking to make sure this sort of tragedy never occurs in the future." One of the union's first aims would be to give all possible help to the families of the victims, he said.
Dennis Scott, operations director at Amec, said: "What has happened has come as a great shock to everyone . . . Until we get the [Health and Safety Executive] reports I am afraid we cannot say too much more about the incident.
"There could be a number of reasons why it happened and I really would not like to speculate before the completion of the investigation."
Mr Scott added: "Obviously everyone is in deep shock and our deepest sympathies go out to the families and friends of the casualties. As a sign of respect we decided to send everybody home so there will be no work in the yard today. It will be closed until further notice."
The four workers who escaped the main force of the blast were all able to go home after hospital treatment.