Need to build a platform 8'x16'x15"...Need Advice

Hello Everyone, I need to build an 8'x16' platform about 15 inches high. I am not a carpenter and I'm hoping to keep this as simple as possible. My idea is to add 3" of height to the 12" joists by adding a 3x3 to the length of each joist plus .75" plywood as the decking. The platform is to support a screened room attached to the side of a trailer on a concrete foundation. Any suggestions would be appreciated. I'm a rookie for certain.