I have outputs from three devices--TV headphone out, receiver headphone out, and turntable out. I currently have an input-selector box between them and the amp. Since I would only have one of them turned on at a time, do I need it? I thought about replacing the selector with a couple of cascading 3.5mm Y adapters. Would this hurt anything?
If you use a splitter the three sources would be connected to each other even if two of them were off. This could load down the active output and might cause other issues. In general you use splitters for making more outputs not more inputs. It probably wouldn't damage anything though.