Officers went to the A390 near St Ive, Liskeard, shortly before Tuesday after a car reportedly veered off the road and caught fire.
The boy and two men aged 18 and 30 were discovered and pronounced dead at the scene.
Two women and two children from Liskeard were later arrested and remain in custody, the force said.
Officers seized a vehicle suspected of being involved in the “tragic incident. “
Sgt. Titus Tina Green, of Devon and Cornwall Police, said: “Our thoughts are with the families and friends of those who lost their lives and our investigation continues to investigate all instances of how this tragic incident occurred.
“While our investigations are ongoing, there will be an increased police presence at the crash site and at the wider LiskeardArray. “
“I’m inspired by data that can also help police and those traveling through the domain and possibly have corresponding dashboard camera footage to inform us. “
Read more: What the Cardiff crash search was like, as police face criticism
He added: “We are aware of some comments and hypotheses on social media and remind the public that there is an active and ongoing police investigation in relation to this incident and please avoid speculating on the nature of the incident and who is involved. “.
“If you see a comment on social media that you think may just be our investigation, let us know as well. “
Joel Corry / Becky Hill
Alanis Morissette
Passenger
Rita Ora