If I am not happy with the hair transplant result, can I have second one immediately?

It is normal to have asymmetries on your hair line, but if it's really disturbing you, you can get a touch up if it's not damaging the other grafts

If there is an obvious asymmetry with the hairline, there is no reason to have to wait for a touch up procedure, it can be done anytime.

It is best to wait until the swelling goes down first.Then you should wait to see the hair follicles to grow back, roughly after 3 months.Also you should allow some time for the donor area to heal and then you can have a top up.
Good luck

During the transplant it is likely that the hairline was correct.  Swelling in the post operative period can make the hairline uneven for as long as two weeks.  Ideally, you would allow all of the swelling to abate before judging the symmetry of your hairline.  At that time, if things were uneven, you could plan for a future revision, if necessary.

Most people who have never had a hair transplant have uneven hairlines and it is natural.  Too much symmetry of a hairline can look quite unnatural-- just look at Senator Chuck Schumer's hairline.  The human eye is drawn to lines, hairlines are not suppose to be lines.
With that said in your specific -- if the you are unhappy with the uneven hairline, you can do another hair transplant now and do not need to wait until the transplanted hair comes out.

First regrowth should be observed on 3-3.5 months after surgery. Regrowing hair is thin, undersized and not bright untill 4-5 th months of hair transplant. İt's normal for new growing hair starting to get native characteristics, waviness, curl in first 6-7 months.so  I advice you  to wait for 5-6 monts before planning a second operation . 

It is not recommended to go for 2nd hair transplant immediately as it's been just 3 days and appearance of hairline may look asymmetrical due to swelling on the forehead due to accumulation of saline water. You need to wait for 6 to 8 months and see how is it growing then decide whether you want a correction or not.

Thank you for posting your question, sure your could go ahead and get the touch-up done anytime within 12-14 days post your surgery or wait-up until 6 months. Feel free to connect with your surgeon and discuss on the possibilities. 

Yes, you could do another hair transplant to fix the imbalance between the two sides. The key is to have no crusting present as that could increase the infection risk