I have some questions about the loss of my dog?

My condolences on the loss of your beloved Cinnamon. I think it's better that your two other dogs saw the body, b/c in most cases, all they know is that their fellow pack member went away and never came back, without knowing why. I think your older dog likely understands the finality of death, as animals other than humans understand what death is. It's fine for them to see you sad, as they are mourning inside in their own way as well. But be sure to enjoy their presence and provide them with plenty of love, fun, and play.