Hair loss after shave: I am 18 years old. 3 years ago I shaved my head.

Shaven head do not cause any hair loss density. You are still young, if you are looking for hair transplantation I suggest you to wait 5 more years. 18 years old it's to young, your hair loss is not stable.

You have to recognize that hair goes through phases of growth and rest.  Androgenic alopecia is a condition where more hair follicles enter the resting phase.  If you shave, the growing hairs become longer and the resting hairs remain short.  This will lead to the appearance of lower density.  This could be your situation. You need to jump start these follicles into growing again.  Inducement of growth is promoted by products like finasteride, minoxidil, PRP, and Wnt-act (amazon store).  Finasteride is more effective topically than orally.  Minoxidil is more effective orally than topically.  PRP is an injection.  Wnt-act is a topical and acts differently than other products mentioned.  Shaving does not cause hair loss.  However, shaving can unmask hair loss.  

Shaving your head did not head to hair loss; most likely the timing was coincidental.  Your best bet is a visit to a hair restorative doctor to discuss ways to slow down and perhaps reverse your hair loss such as platelet-rich plasma (PRP), laser, finasteride (Propecia) and minoxidil (Rogaine), andthen once things are stable consider a transplant. 

First, it is really important to understand that shaving does not have any role here in changes in hair density. If an individual does not have a hair loss condition they can shave their hair as often as they like and they will not experience any long term issues. You may wish to see a doctor for advice. To diagnose hair loss, a physician will need to see your scalp up close and review your story in more detail.

Shaving does not cause hair loss, but in your age range you may have had additional genetic hair loss in the time it took for your hair to regrow.  Hence the difference you noticed from when your hair was grown out before.  Try finasteride and minoxidil.  In your age range you are likely to be able to regrow everything you lost.
Good Luck!

If you shaved your hair and it did not come back normal, then most likely you have genetic hair loss and what you are seeing is the hair loss from that condition, see a local hair expert