Is the 3.46 your degree-granting GPA or the one LSAC calculated for you? Regardless, from what you've said it doesn't sound like there was any huge dip or surge in your performance--you went from As, to As and Bs, with a C in there second semester. In this case, I probably wouldn't write an addendum. You might use your illness as a PS topic, if you want to do a "challenges overcome" sort of statement, in which case you'll have alerted them to the illness as a previous factor in your academic career. But your grades are really pretty stable. If you'd gone from As to Cs and Ds and then back to all As again, then I could see an addendum.