why is my ecobee thermostat reading 3 degrees hotter than every other thermometer in my house?

Warm air from the wall cavity is influencing the sensor. Or the sensor needs calibration.

First, seal the hole behind the thermostat where the wires come through. Use plumber's putty. This stops wall cavity air flow.

Then, calibrate the sensor.

1. Get a trusted thermometer. Place it near the thermostat.
2. On the ecobee, go to MAIN MENU > SETTINGS > INSTALLATION SETTINGS > THRESHOLDS > TEMPERATURE CORRECTION.
3. Adjust the offset to match your trusted thermometer. You can correct by +/- 10°F.

If that didn't work, check if you have SmartSensors paired. The thermostat reading is an average of all participating sensors. Go to MAIN MENU > SETTINGS > SENSORS to see the list. The thermostat's own sensor has a variance of +/- 1°F.

Heads up: Only compare the thermostat sensor to your thermometer when validating. Sensor average readings are different.