Problem is that you reassign the same variable "floodSensorState", so it's value will always be the value of digitalRead(floodSensor3) (since it is the last assignment).
You could do something like that:
if ( digitalRead(floodSensor1) || digitalRead(floodSensor2) || digitalRead(floodSensor3) )
// turn LED on:
// turn LED off:
|| mean "or"
It is also possible, if all the floor sensors are on the same PORT, to check if the corresponding bits of that port are set, with only a
few bitwise operations
single operation. Which Arduino do you use?