What are the 3 fundamental rules to be followed while servicing in Badminton?

In no particular order:

1) Positioning of body, bird, racquet - your body should be relaxed and stable.

2) To transfer their weight from back to front using power generated from whole body ending with a wrist flick. (and a follow through across the body for underhand serve)

3) To make contact with the shuttle below your waist with the racquet head below the wrist.

4) To have the proper grip for their serve. (Forehand grip/Backhand grip). Grip should be relaxed. For maximum control, hold the top part of the grip.

5) To not lift their back foot from the ground.

6) Bonus tip - take your time and a deep breath - most bad serves are because the server has rushed himself and not focused on the task.