How can I get fat enough to go up three clothing sizes?

You eat whatever you've been dying to try but been holding back on for so long.

But DO NOT eat until you faint or something. Just eat a McDonalds burger, once a week or something like that. Chips, chocolate, you name it.

Just... Eat. Don't barge. Until you get up to your size, then continue your regular diet, just with a little treats at the end of the day.

Go to the grocery store and buy as much as you can of anything that says "diet," "sugar-free," or "low fat" on it. That crap makes sure that fat people stay fat, so it's bound to help you. You can further damage your health by making sure that 100% of what you eat comes from processed food or fast-food restaurants.

But seriously, why would you want to do that to yourself?

If you have trouble gaining weight and are underweight your doctor can put you on medication to improve your appetite like Megace.

That wouldn't be healthy to do.