I haven't lost any weight in 3 weeks. I had my gastric bypass surgery 2 months ago

Your metabolism may have slowed.  Now that your weight is lower you have a lower BMR, the very low calorie postop diet also affects metabolism.  When you start exercising focus on building muscle.  Building muscle will increase metabolic rate, muscle also weighs more than fat so pay less attention to weight and more to body fat percentage, body measurments or clothing size.  In your diet pay special attention to getting the best possible nutrition from food you can eat.

I am 6 months out from roux-en-y gastric bypass. I have had plateaus as long as 1 week to over 4 weeks. It does move again. But it is important to examine your habits during this time as well. You may lose all the weight you want to and then some without any exercise at all, and with not following the rules, but you can bet if you do not implement these needed changes, you will NOT be successful at keeping it off over the long term.

My weight loss surgery path has show losses in steps.  I would plateau often,but then lose a large amount quickly.  It wasn't until I reach 100 lbs. lost, that things really started to slow for me.  Then I found out I am hypothyroid. :(

At this point you should still be following up with your surgeon, or PCP who has experience caring for Bariatric patients.  If you are not, then find someone. 

Most of all, don't compare yourself to others. I know it's easier said than done, but everyone is different.