What are south asia's three largest rivers and what are their source?

The three largest rivers are the Ganges, the Indus , and the Brahmaputra . 

The Ganges has the second greatest water discharge in the world and the basin is is the most populated in the world with over 400 million people living there.

The Indus drains one of Asia's chief catchment basins, and is the locus of some of the most earliest known human civilizations. It is the twelfth largest river basin on earth.

Im not sure about the last river but hope this is what you needed x 

Alexis x

The three largest rivers in South Asia are -Indus River (3200km long). This flows through Pakistan, India and China. 93%, 5% and 2% respectively in each country. The main sources of this are the Sênggê River, Tibetan Plateau and the Gar Rifer -Brahmaputra River (2900km long). This flows through China, India and Bangladesh. The main source of this is from the Himalayas. -Ganges River (2525km long). This flows through India and Bangladesh. The main sources of the Ganges are the Satopanth Glacier and the Gangotri Glacier, and the water from some of their peaks, such as Trisul, Nando Kot, Nanda Devi, Kedarnath and Kamet. Hope this helps