my check engine light blinks when i go over 30 mph..runs kinda rough and stalls on 2003 Chevrolet Impala

took it to auto zone and they said it was the #3 cylinder i changed fuel injector and still blinks over 30 mph

The 'blinking' check engine light is a sign that your engine is running poorly enough that it IS doing damage to your converter (and most likely other systems as well).If you're doing the 'repairs' yourself , spend money on a correct diagnosis before buying more parts. Codes tell you what system is being affected many times , not that a specific part is bad (although many do).DO NOT HAVE IT DIAGNOSED BY A PARTS PLACE. They sell parts. A repair shop diagnoses and repairs cars.
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3 more answers , 4 more comments
did you repl plug and coil also. what lead u to repl inj 1st. they are almost always good over plug and coil
they said that the injector might be bad so i replaced it and the plug and coil..but it still confuses me because even if i put the car in neutral and coast down the hill..right at 30mph the check engine light starts blinking
Most likely coil or spark plugs.
A misfire is almost always caused by not enough fuel or spark. You will need to determine which is the cause.
the spark plug was changed...the old one had black oil on it looked like
re-check the new spark plug after a few hundred miles and see if you are still getting deposits on it.Make sure you got OEM type plug! Reply with what you find.
still does it....plug is still oily lookin