The three principal river systems running through this region are the
bangla
ganges
brahmaputra
indus
terai

The correct answers are:

- Ganges;

- Brahmaputra;

- Indus;

South Asia's three biggest and most important river systems are the Ganges, Brahmaputra, and Indus. All three of them all located in the northern half of the region.

Indus is running mostly through Pakistan, in a roughly north-south direction, Ganges is starting in a north-south direction, making a turn towards the east, and ending up in north-south direction again, while Brahmaputra is mostly snake like in its flow, and it is going from north to south in general.

These three rivers are the ones where the first civilizations arose in this region,  but nowadays they are all heavily polluted and neglected.