‘My ecobee3′ is difficult for Alexa to understand. Knowing this, if the name of your ecobee is ‘My ecobee3′, we’ve adjusted it to ‘ecobee’ for Alexa. Try saying, “Alexa, set my ecobee to (75) degrees” and Alexa will now understand.
To change the name of your ecobee, go to Settings > Preferences > Thermostat > Input the new name. Once you’ve changed it, you’ll need to ask Alexa to discover devices again.