One of the trucks smashed into the rear of the other, which had rear-ended the bus. The crash happened around noon Monday on westbound I-96 in Ionia County's Berlin Township, state police Lt. Brady Boucher said.
Another wreck had occurred earlier and police were clearing the freeway when the trucks and bus crashed, Boucher said.
Jordan Wilson said that he was on the Grand Rapids-bound bus when he "felt a bump."
"There was a crash up ahead, and the driver started to pump the brakes to try to slow down," said Wilson, of Grand Rapids.
About 25 people were on the bus, he said.
The 41-year-old truck driver who was seriously injured was flown to a Grand Rapids hospital. He was in surgery Monday afternoon.
One bus passenger suffered minor injuries and was driven to an area hospital, Boucher said.
A portion of westbound I-96 in Ionia County, about 20 miles northwest of Lansing, was closed for several hours after the crash. Traffic backups as long as 10 miles were reported.