@corvetteii, I’ve already coded several smartapps that can interact with any ST connected temp & motion sensors and the smart vents:
(1) ScheduleTstatZones
See
(2) I recently created a simpler version of the smartapp called ScheduleRoomTempControl.
See the ST community wiki for more details (and specifically the configuration section):
http://thingsthataresmart.wiki/index.php?title=Category:Unpublished_SmartApps
So, you will have different options depending on your use cases & requirements.
Regards.
P.S. I have tested them at my place with several ST connected temp & motion sensors and they work well.
EDIT: If you have issues with the ecobee3’s remote sensors, just try to use MyEcobee Device w/o the remote sensors exposed to ST for a while (i.e. uninstall ecobee3RemoteSensorInit). You can then verify whether or not the connection issues are more related to the remote sensors per se.