You seem to be equating food weight with body weight, and they are not directly related.
Yes, if you eat a pound of something, you will immediately weigh one more pound, as your body has not had a chance to digest it and process it as needed. However, that doesn't mean that you will have gained one permanent pound. The body will break down the food, distribute the end result to various places for either use or storage, and get rid of whatever is not digestible.
Weight fluctuates during the day, so the best gauge of your weight is to weigh yourself at the same time every day, under the same conditions. Track that number, and that gives you your true weight.
Also, weight gain/loss is a relationship between how many calories you need to sustain your day to day activities, and how many you eat. If you consistently eat more calories than you need for a day, then you will gain weight. If you consistently eat less, then you will lose weight. The rate at which you do so varies on how big the deficit/surplus is, how efficient your metabolism, type of calories, many factors such as these.
For your main question, 3500 calories per pound of food is very calorie dense. For example, 1 lb of peanut butter is going to be ~ 2600 calories. The higher the fat content, the closer you get to that mark. If you ate a straight pound of fat, for example, you would get just over 4000 calories. (Using the typically accepted 9 calories per gram of fat, which is also not 100% accurate). So yes, it is possible to get more than 3500 calories in a pound of food.