Losing power when accelerating, then flashing check engine light

I have a 2003 Chevy Cavalier, and here recently, i've been losing power when I accelerate, and my check engine light comes on and starts flashing. The only thing that's popped up on the OBDII is Cylinder 3 misfire. I changed spark plugs, and it helped for a day or two, but then went back to the same thing. Today, a loud clacking noise started coming from the engine when i accelerated, and i could feel it in the gas pedal, and barely made it home. Help?