Sinus pressure behind l eye before and after surgery. no infection. all imaging is -. 3 antihistamines 3x/day helps. is it bad to keep taking these?

Won't work: you need re-evaluatin, see your ENT or another ENT, second or even third opinion, it's your right. I don't know what kind of surgery you had, but constant pain like you described would not be solved by just tripling antihistamine dose, good luck

Sinus pressure : Be sure to have your eyes checked as well: get pentacam to be sure angles are open: are you on Topomax? And get refraction. How much antihistamine are you taking?