I need 3 simple boxes coded in CSS3 to be responsive for desktop, mobiles and tablets

I am currently building a website using WordPress. The theme I am using is fully responsive and displays nicely by changing its layout depending on where it is being viewed from; desktops, mobiles or tablets.

I am now adding in my own code and styling elements, but have no experience in the CSS3 needed to make this fully responsive.

I have attached a file, explaining what needs to be done and the code must work for all browsers (any backward compatibility that can be done, would also be fantastic).

Please fire away with any questions. If this can be turned around within 24 hrs, that would be great.

Thanks
Michael