Rough idle and runs rough until it reaches thirty to forty MPH after which the check engine light starts flashing

My vehicle has a very rough idle and when I take off it's very rough. When I reach about 30-40 MPH it's fine and runs smooth. However when I go past 40 MPH the check engine light starts blinking then it leaves a code. I read the code and it says cylinder 3 misfire. I changed all 3 ignition coils, wires, and 6 spark plugs. My fresh air intake hose was ripped so I replaced that too. I test drove it and still the same behavior. Very rough idle and rumbling as I speed up and blinking check engine light when I speed up. I have no idea what else to do. Also I replaced all 6 fuel injectors about a year ago and replaced all the gaskets on the plenum. All this started happening after I changed the oil and oil filter. I have no idea why this is happening. Any help would be great. Thank you