Finding the "C" Wire on old Honeywell L8048G Controller/Relay
It’s possible you don’t need a C (common neutral) wire when installing your Ecobee3. Check out the guide on using the Power Extension Kit that comes with your new thermostat: https://www.ecobee.com/faq/i-do-not-have-a-c-wire-or-common-wire-can-i-install-ecobee-residential-thermostats/