2ry haemorrhage (subdural haematoma < 72 hr. acute SDH): This is the most likely cause of loss of consciousness in a patient with a history of RTA, scalp laceration, facial, and femoral fractures, who becomes unconscious on the 3rd day. The timeline and the nature of the initial injuries are consistent with the development of a subdural hematoma, which can expand or become symptomatic due to increased intracranial pressure.

Fat embolism: Given the presence of femoral fractures, fat embolism syndrome is a plausible cause of loss of consciousness. Fat embolism can occur after a delay following long bone fractures and can lead to respiratory and neurological symptoms.
Electrolyte disturbance: Although less directly related to the initial trauma, electrolyte imbalances can occur due to various factors such as fluid resuscitation, bleeding, or renal issues in the context of severe trauma. Significant electrolyte disturbances can lead to altered mental status.

SAH (Subarachnoid Hemorrhage): While less common than subdural hematoma in the context of trauma, SAH can occur and is critical to identify due to its high morbidity and mortality. It requires prompt diagnosis and treatment.
Infection or Sepsis: Infections can complicate trauma and lead to sepsis, which can cause altered mental status. Early recognition and treatment are crucial.

Other intracranial hemorrhages (e.g., epidural hematoma, intraparenchymal hemorrhage): While these are less likely given the specific timeline and presentation, they are possible and should be considered, especially if other diagnoses are ruled out.
Venous thromboembolism with paradoxical embolism: This is a rare but potentially fatal condition where a clot forms in the deep veins, breaks loose, and travels to the brain, causing a stroke. It's less directly related to the loss of consciousness in this context but should be considered in the broader differential diagnosis of post-traumatic complications.