Smoke & Carbon Monoxide Sensors

You're right, SS2 components/sensors do not work with SS3.  I have neither the smoke nor CO detectors so can't tell you what is different in appearance (they also look the same on the order page here:  https://simplisafe.com/alarm-sensors for both versions), but if you've tried to enter them into the system and it didn't work, chances are, SS could have sent you the wrong versions - call them to get the correct ones.

The old sensors are not encrypted and the new ones are encrypted.  No way they can communicate in the same system.