hmm He's only 3 - kids forget easy.
Is the tv on the floor, in an entertainment center? Can you put a small bench or toy chest or something in front of it so he can't get close? A special chair just for him that is far enough away? Maybe put that blue painter's tape as a "guide" of how close is too close? Ha, it'll look funny but trust me in all parents homes you can find something funny and say "oh yeah! good idea!" If he gets in the perimeter of 'too close' remind him that he's supposed to be on this side of the line.
I'm a big believer in positive reinforcement, catch him sitting the right distance and go nuts praising him "WOW, look how nicely you are watching tv from the chair/couch/whatever! I'm so proud of you!" or something silly like "Hey Mr. Eye #1 appreciates you sitting away from the tv so he doesn't get a headache!"
Too much praise for something like that may seem silly but it will let him know you notice when he is doing something right.
When my kids did things they weren't supposed to I'd try different tricks like for him maybe I'd say "I'm sorry but when you sit too close I have to turn the tv off so I don't worry about you hurting your eyes. Maybe when you're ready to sit on the chair over here we can turn it back on. I love you an dI don;t want to worry about you hurting those handsome eyes of yours."
Stay consistent... your child is lucky to have a parent that cares! My friends have made fun of me for silly things I do but the funny thing is I catch them doing the same thing! My son wouldn't take his medicine when he was about 3 so my hands transformed into "Mr. Fitzgerald Hello" and "Mr. Winston Hello" and they would always 'taste' the medicine to make sure it was okay first. My mother in law laughed at me but the next day when I picked my son up after work she said there were problems getting the meds down until her hands became a Mr. Hello also.
=) Good luck