New build doesnt turn on

[Nextgenthebear]

i put together this build but with a different power supply from my old computer and its a 450 i plugged everything in but this one 3 pin that i couldnt find a place to plug it in and i dont even know what it is its apart of my case, ive tried two different power cables and they both work on my other computer. would anyone know if its the power supply or something else? like nothing turns on. https://pcpartpicker.com/list/V4JxTH

 

[popatim]

soulds like your old computer used a proprietary PSU (dell, hp,...) and you'll be buying a new one now. Sorry to say 🙁