Give me the code for a webpage using css, html and javascript. The page should have 3 buttons with rounded corners in the middle of the screen that are centered horizontally. The buttons should have centered text that says "click me". The buttons should have a lime color. When one of the buttons is pressed it should bring a random alert message from the list below:

1. Hello there!
2. How are you doing?
3. I'm here!
4. Wobba Dobba!

The background of the page should be a bright gradient yellow color.

Please combine the code into one so it's easy to copy and paste. Please use JSON format in your answer.
user