Guild Wars 2

I had a 3-4 week break from gw2 and these achievement chests confuse me. i have 3500+ AP but i dont know how do i get the 3000 AP chest

You obtain a chest every 500 AP. However these chests will be awarded once per day. You’ll eventually get it.

If you hover over the chests in the achievements panel, you´ll see that it says retroactive chests are awarded once per day. So you´re going to get one chest every day until you´ve gotten to your 3,500 one, which should take 7 days in total.