I wouldn't add more leg work to the 5x5. Doing squats 3 times a week and heavy deadlifts, your legs will be pretty loaded. I guess you could throw in a couple sets of calf raises on one day if you wanted.
I would also recommnd that you start light for week one and adjust the weights from there, as this is your first 5x5. You don't want to be near failure on any set in week two. Set some resonable goals for week 4 and 9. Plan each week to lead you to those goals. If you think your goal is too modest or too high, then adjust you rit slightly. Try to make it so that you hit your 5 or 3 reps, but you have about a half rep/rep left in you. On week 9, ideally, you want to do 3 with the last one just at failure.
This is my view of the 5x5 and I think it will make it more enjoyable. Failing to hit your reps early will be mentally/physically draining. Going up every week in weight provides a lot of postive reinforcement. Even though you will be dying to max out or see what your real max for 3 will be, have patience. Follow your program and by week 9 hopefully you will be very happy at the results.