MacBook Pro 2015 lost battery capacity I have used my new MacBook Pro Retina for a few days.

After three battery cycles, the battery lost capacity from 6960 mAh originally to 6616 mAh now.

Is it normal to lose almost 350 mAh of capacity in just two/three days?
As per this wikipedia article, 


  Batteries gradually self-discharge even if not connected and
  delivering current. Li+ rechargeable batteries [like the kind in a
  MacBook] have a self-discharge rate typically stated by manufacturers
  to be 1.5-2% per month.


Basically, even if you are not charging or discharging your Mac battery at all, it will still lose 1-2% of its capacity every month (on average). You stated your battery age as 7.2 months, and your original battery capacity as 6960 mAh. (Side note: anyone else can find out their battery capacity, original capacity, and battery age with this tool.) Assuming it discharged 1% a month while in storage for 7 months, that means your battery could have lost 487 mAh while in storage. So, it is perfectly reasonable for your 7 month old battery to have lost 350mAh. 

It is a little frustrating to have a somewhat old battery when you have purchased a new computer, but that's just how it goes. Li-ion batteries last well for about 4-5 years, so 7 months isn't too long on that scale. You will probably need to replace your Mac's battery (or get a new computer) in 2021. So, you have plenty of time to enjoy your new Mac before things even start to go downhill. But yes, that small battery discharge is normal; remember that it occurred over 7 months, not 3 days.