Final answer:
The Third Druk Gyalpo established the National Assembly primarily to transition Bhutan towards a constitutional monarchy, fostering Gross National Happiness and extending democracy.
Step-by-step explanation:
The main objective of establishing the National Assembly by the Third Druk Gyalpo of Bhutan was to lay the groundwork for a constitutional monarchy, ensuring the welfare and happiness of the people based on the principles of Gross National Happiness. It was a transition from centuries of absolute monarchy to a system that allowed for more democratic participation and provided a broader base for national wealth. The establishment of the National Assembly aimed to create a constitution and laws that would reflect the collective will of the Bhutanese people and offer more direct control over their government as part of the country's modernization efforts.