Three cushions make sense. I'd suggest two inch foam. Measure how high this would be compared to your dining room chairs.

I also would suggest a very dense foam and 3 cushions and best in a outdoor type fabric.

Will you be using this with a dining table, or just occasional seating? If the former, then you need to make sure that the height of the cushion works with your table height. Setting that aside, I belive the standard is 3-4" for something that is comfortable to sit in for extended periods. I don't see why you wouldn't use three cushions and it may come in handy - if you stain one cushion, you'll only need to recover that one vs the whole thing. I have a built-in upholstered dining bench in my kitchen and I recommend a down wrap over the foam - much more comfortable. I find myself sitting on the bench to do work or just relax, even when not dining.

Thank you for your feedback. It will be for occasional seating not for dining.

WIth three, you can flip them and rotate for more even wear. In addition to considering the foam height, you need to consider the foam density. You can have a 3" foam that squishes down to 1" when you sit on it, or a 3" foam that hardly compresses when you sit down. Because you will not have a table hiding this area, I would go for a 4" foam. It will create a more substantial scale. The density is a matter of preference. My back loves dense foam and no down.

@cableknit Do your cushions slide around much? Do you use anything to keep them in place on the bench?