Tub Rough Opening 63" - Fill the gap for 60" Tub

Hi, I'm remodeling the bathroom in my house and am installing the bathtub. My bathroom's rough opening width is 63". When I install my 60" tub, there is still a 3" gap on the left side of my right drain tub that needs to be address. I'm at a loss for how to do this. Should I use Kerdi board and make it a shelf? Should I fill it with wood and then tile over it? Any help or thoughts would be greatly appreciated! Here's a photo (https://dl.dropboxusercontent.com/u/7348551/Upload.jpg) of the current setup Thanks a lot!