Two females from one vehicle and both drivers from the other two vehicles involved were taken to the ERI after the accident at 9:30pm, at Nine Mile Burn, near Penicuik.
Another male was treated at the scene. The extent of their injuries is not known.
It is believed that bad weather conditions played a part in the incident.
Three fire engines and a metal cutting appliance attended. The females, aged 36 and 65, had to be cut free from their vehicle.