2005 cadilliac srx 3.6. computer said number 3 misfire changed it, car is idling on 2005 Cadillac SRX

rough, replaced all spark plugs, thought maybe water was in tank bought water removal from autozone, new fuel filter, when driving down the road car starts to shutter in the back, I'm stumped help, any help would be appreciated. already has a new transmission.

Asked by for the 2005 Cadillac SRX
Water in gas wont pick out only one cylinder to misfire! More likely coil over the spark plug or injector problem. Can swap coil to see if misfire follows the coil. Or see a mechanic.