The incident on Nakuru-Eldoret Highway, involving a Siaya county vehicle, took place at about 1.30pm.
Two people died at the scene and the third on the way to Nakuru Level Five Hospital.
Ziro Arome, Rift Valley provincial traffic enforcement officer, said the driver of the county car had been speeding and lost control when he reached the bay.
“The vehicle was from the just-concluded 4th Devolution Conference in Naivasha,” he said, adding the trailer was on its way from Mombasa to Eldoret.
Wesley Kimutai, a witness who sustained a minor injury, said: “The trailer’s driver was supervising a tyre’s repair. I heard a loud bang and realised he had been knocked down.”
The county car then hit two people who were walking along the road.
It was taken to Salgaa police station for inspection and the bodies to the Nakuru county morgue.
Many accidents have been reported in the Molo General Service Camp, Sachang’wan, Migaa and Salgaa areas, which are blackspots. They have been linked to speeding and illegal overtaking.