My best advice to you, would be to get a multimeter, start from the fan and work your way back to the fan switch sensor.
Once the vehicle is warmed up, check for current to the fan. If there is power going to the fan, the fan is bad. No current, head back to the relay.
If there is power going to the relay, the relay is bad, No power, head back to the switch sensor.
The component itself may not be bad, since you have replaced them all, but perhaps there is a bad connection somewhere in between the two components.
The only other things that i can think of, is that there is not water/coolant getting to the temperature sensor. Make sure that you are full on coolant, your water pump is working properly, and the coolant is flowing through the engine. Even a bad thermostat may prevent water from getting to the sensor.
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