Re: need to gain weight..
I am familiar with the weight-gaining process myself and what I can tell you is that it is a lot harder than you may anticipate to do, especially if you are active. Do you exercise?
First of all, stuffing yourself with junk like donuts all the time is not the way to gain healthily. Of course, sweets now and then won't hurt you; the extra calories can help. You do need to incorporate nutrient-dense foods so you gain muscle mass and not all fat. If you light weights, that will help. Some foods you may want to consider: peanut butter sandwiches (whole wheat bread - and jelly is fine but look for low sugar/no sugar added kinds), lean meats, potatoes, nuts/seeds, dried fruit, Ensure drinks, extra servings of fruits and veggies throughout the day, bars, granola...
What are you eating now? Every 3 hours may not be enough to make you gain, since your metabolism is working off those calories all the time and is faster if you are active. You need 500 extra calories a day, that's 3500 a week to gain one pound a week. Look up the Harris-Benedict equation to find a ballpark number of calories you burn in a day, considering the amount of physical activity you do, and go from there.
By the way, gaining 1-2 pounds a week is healthy; don't stuff yourself thinking you can gain 10 that quickly. Besides, more than 2 pounds a week is probably just water weight.