I'd imagine they'd drop PS3 support at the end of Heavensward. And that isn't to sound callous, because I was on PS3 for all of ARR and couldn't afford a PS4 for the longest time. But even then, I was aware that the PS3 was holding the game back.
I've tried several times to get comparison screenshots to see what Heavensward looks like on PS3 versus PS4 or PC, just for curiosity's sake. I can't find any, anywhere. Most sites barely acknowledge that the game exists on that console, which leads me to think it must perform very poorly on there. I ran into a PS3 player in Snowcloak not long ago, and she couldn't see AoEs from the final boss because the snow effects would make her lag so badly. I can only imagine what Western Coerthas would be like...