Losing 10 pounds in 3 weeks could be easy or difficult depending on what your starting point is.
If you have a lot of weight to lose, it can be quite feasible though.
3.3 pounds of weight loss/week.
A good amount of weight loss to shoot for normally is 1-2 pounds/week.
Either way, what you need to do is cut your calories...just don't cut them lower than 1,200 calories/day.
Couple that with exercise and don't stray off the diet.
You'll want to cut out all processed foods you're currently eating in your diet as they tend to be high in calories and don't provide much satiety.
eat a LOT of vegetables...lots of salads (watch the dressing though, go for light Italian dressing). Also eat lean meat and some fruit...
Other foods to consider are nuts(not too many, they are calorie dense), oatmeal, brown rice, whole wheat bread, yogurt, no fat cottage cheese, eggs.