I have recently had FUE treatment Day 12 3500 grafts. Do you think my hair will be thick enough looking at my pictures? (photos)

It can take 6 to 12 months after surgery to see results.  If you are concerned it is best to contact your doctor

With 3500 grafts, your hair should be thick enough. From the photos, your hair looks normal. Consult your surgeon if you continue to have pain.

It looks good post operatively. You need to wait for at least 6-8 months to see a change in your hairloss and 12 months for a final result. Hang in here.

You look fine for being only 12 days out.  Unfortunately the scabs will take another week or so to fall of completely.  You may need to conceal as best you can until then.

Its very difficult to see the numbers, the area or type of hair you have from the photos supplied, in my experience anything over 1500-1750 grafts on an average patient is the most that can be harvested successfully, without over-harvesting your donor site.
I often see patients who have been told a number and don't fully understand what that number actually means. It may be your 3,500 is just hairs, not grafts, and there is a big difference between the two.  Ask your surgeon for the surgical report, that should give the right information; confirming the number of single, two and three or more hair grafts. An average of 2.4 hairs over a surgery per graft is normal. So your 3,500 would possibly be more like 1,458 actual grafts.
This number would normally give a reasonable result, with an experienced team.
Good luck. 

It looks ok to me; however, you will not know for at least 8 months from the time you had it done when the hair will grow to styling length