I prefer to focus more on two cases and link in a third sporadically. The afterword in particular works well for this, because it sums up Robertson's entire perspective on everything else and links all the cases together. Your structure mostly depends on what sort of English student you are- whether you prefer to have a more interwoven approach or a conventionally organised essay. I'll usually have a sort of basic structure to work with, but I find that sometimes interrupts the flow of an essay.