How did i gain 3 pounds in a week when diet stays the same and even added power yoga.

Several possibilitie: You may have added muscle which increases weight. You may have eaten more than you think. You may be retaining water for some reason such as having your period, etc. Don't get nervous about a small short term weight gain. Make sure your diet is a healthy one that will eventually result in healthy weight reduction. Exclude simple carbs and saturated fats found in pasta, breads, cakes, creams, etc.