Thursday's crash happened at a level crossing in Skogby near the southern coastal city of Raseborg, about 85km southwest of Helsinki.
Police said four people were killed, including one train passenger. A Finnish general confirmed that three soldiers died.
"Today I have received a heartbreaking message: Three conscripts have died in an accident in Raseborg," General Jarmo Lindberg said in a statement.
Finland's defence minister, Jussi Niinisto, tweeted that Thursday "had started with grim news" and "I feel grief."
The FNB news agency says eight other people who have been injured in the crash have been taken to hospitals in Raseborg and in Helsinki.
The military truck was carrying 11 soldiers, who were taking part in a drill in the southern part of the Nordic country.
Images from the scene showed a slightly damaged train and the wreck of what appears to be a military vehicle in the woods. The train was standing upright, but it wasn't immediately clear if it had derailed.
Finnish broadcaster YLE said Niinisto had ordered flags at all military facilities to fly at half-staff on Thursday.
Finnish President Sauli Niinisto expressed his condolences in a statement, calling it an "unfortunate tragedy," saying the conscripts were taking part in "a task for the motherland."
The wreckage of a military truck lays by the side of the railway tracks after several people were killed in a crash with a train in southern Finland (AAP) Source: AAP
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