I have done my FUE 3 weeks ago, and noticed the hair line the doctor put is very down.

There is no way to reverse what was done.  A great surgery will be with you for life. Likewise a  bad surgery is for life.  Surgery should not cause life long headaches.

Drawing a lower hairline is the worst outcome of hair transplant in the later age...
Draw a straight line from one mid eyebrow point to other mid eye brow point and your mid frontal hair line point( a cebter and front nost poibt in your hairline)hairline should be at least 7 cm above from this...

The hairline design needs to be age appropriate and also look natural as you mature in your years (in for 40s, 50s, 60s, etc).  In general, the hairline should be positioned  at least a finger breadth (approx 2cm) above the highest frown line on your forehead.  This is where aesthetic judgment and dialogue with your surgeon needs to happen to make sure you can visualize what you may look like.  Detailed macro and micro irregularities along the hairline then make the hair line look natural.     

The way you find out where your mature hairline belongs is lift your eyebrows high so that the forehead creases. The mature hairline is found one finger breadth above the highest crease. A V shaped then brings the hair to the corners