Authorities say the van somehow lost control and crashed around 2:27 p.m. on Route 1 just north of Newark Road in New Garden Township, Pennsylvania.
One person was thrown from the vehicle. Two others were trapped in the van and had to be extracted by fire rescue teams.
Nathan Royal, 44, Thomas Lopez, 61, and Liking Soeung, all of Philadelphia, died in the wreck. Eight others are being treated for unknown injuries.
Some of the victims may have ties to the snack company Herr's.
"It is our understanding, although we have few details at this point, that this vehicle may have contained employees of a temp service we used. We have no other information at this time. They were not full-time employees of Herrs, and this is all we know at this point," a spokesperson for Herr's said.
A 5-mile section of Route 1 has since reopened.
The cause of the accident remains under investigation.