A man aged 36 is the third to die as a result of the crash on the M4 on Monday morning.
The incident involved a van and a lorry and took place on the westbound carriageway close to junction 17 at Chippenham shortly after 4.30am.
Two men travelling in the van were pronounced dead at the scene and two other men, also travelling in the van, were taken to Southmead Hospital in Bristol with serious injuries.
One of these men died from his injuries last night.
One of the three deceased has been formally identified as Martin Williams, 36, of Trecynon, Wales.
Formal identification of the other two is likely to be concluded in the next few days.
The remaining victim remains in hospital in a critical condition.
A fifth male, who was driving the van, was taken to the Great Western Hospital, Swindon, with minor injuries but was released the same morning.
A 37-year-old man from Aberdare was subsequently arrested on suspicion of causing death by dangerous driving. He has been released on police bail pending further enquiries.
The M4 westbound was closed for a total of nine hours while recovery of the vehicles took place.
Anyone who witnessed the collision is asked to contact Wiltshire Police on 101 or Crimestoppers on 0800 555 111 where information can be left anonymously.