Can I use SmartThings Motion Sensors w an Ecobee3?

Yes, you can use any ST connected sensors and I have a smartapp to expose the ecobee’s remote sensors.

Please note of the following known issue with motion capability of the Ecobee’s remote sensors:

http://thingsthataresmart.wiki/index.php?title=Ecobee3RemoteSensorInit#Known_issues

Refer to my thread for more details:

Regards.