on dying hair

If you did it three years ago, the dye should've all grown out and been cut off at the ends before now, unless you either have super-long hair or super slow-growing. Are you sure you're not just thinking it's still in there due to self-consciousness or something?
Anyway, once you've lightened dark hair, it won't just fade out, because the dark pigment was taken out when your hair was lightened. The only way to make it dark again if it's light is to have pigment put back into it. I've never tried to darken bleached hair myself, so I don't know by experience, but I've heard many horror stories about attempts at home dye causing it to turn green, so apparently it's really best done in a salon.