How the heck did this end up causing me to gain 3lbs!?!?

Well weight fluctuates about 5 pounds throughout the day so i don't think you gained any fat really. Also the sugar and salt in what you ate could have caused you to bloat (more water weight). I'm sure on that strict of a diet a little indulgence wouldn't do that much damage. You probably only took in an extra 1500 to 2000 calories max. Drink a lot of water to get the sugar and salt out of your system and i'm sure you'll get back to normal.