Using earpods from iphone5 with iphone 3g

You cannot directly plug your headphones into the Audio Input jack and have it work, because, the microphone/button signal rides on the third ring, and the audio input jack only connects the first two. You have to have specially designed hardware to connect to that third ring. This is currently achieved via the Headphone/Audio Output ports of devices.

Only the iPhone/iPod Touch/iPad, 2010 MacBook Pros, and the modern body (smaller height) Mac Minis are the only pieces of hardware I am aware that do. Anything older will not support the microphone/button/volume buttons at all.

I just checked; the "play/pause" button works, but not the "up/down volume" buttons.