Can too low oncotic pressure lead to Third spacing and Edema?
Yes it can! Its a very important cause of third spacing a.k.a. peripheral oedema, mainly due to decreased protein. 

Below is a list of causes of peripheral oedema due to fluid moving from the vascular bed to the peripheral spaces gathered from the following source: http://www.patient.co.uk/doctor/Peripheral-Oedema.htm

Immobility: Increased fluid pressure from venous stasis. Varicose veins.

Obesity: Increased fluid pressure from sodium and water retention; should not to be confused with non-pitting lymphoedema.

Cardiac: Increased fluid pressure: right heart failure, constrictive pericarditis.

Drugs: Increased fluid pressure from sodium and water retention: calcium antagonists, non-steroidal anti-inflammatory drugs (NSAIDs), prolonged steroid therapy, insulin.[1]

Hepatic: Decreased oncotic pressure: cirrhosis causing hypoalbuminaemia.

Renal: Decreased oncotic pressure from protein loss, and increased fluid pressure from sodium and water retention: acute nephritic syndrome, nephrotic syndrome.

Gastrointestinal: Decreased oncotic pressure: starvation, malabsorption, protein-losing enteropathy (eg Crohn's disease, ulcerative colitis, tumours of stomach and colon, coeliac disease and other intestinal allergies).

Pregnancy: Increased fluid pressure both from sodium and water retention and venous stasis from pelvic obstruction.

High-altitude illness: Oedema of face, hands and ankles may occur.

Idiopathic oedema: Associated with cyclical high lymph volume overload, or dynamic insufficiency: usually in a woman aged 20-40 years. Variable and not related to menstrual periods. Diagnosis is based on the exclusion of other causes of oedema.

Post-thrombotic syndrome: Late complication of deep vein thrombosis (DVT) which occurs in up to two-thirds of patients. May present with pain, oedema, hyperpigmentation, and even skin ulceration. May result from remaining venous obstructions, from reflux, or both. Rate of reflux is highest during the 6-12 months after an acute DVT. It may be temporary and self-limiting or not resolve and persist at variable severity.

Pitting localised limb oedema:

DVT: Compression of large veins by tumour or lymph nodes. 

Following hip replacement or knee replacement. 

Local infection, trauma (including burns, which may also cause generalised oedema because of protein loss), animal bites or stings.