I also have 3 Echoes but they all use the same profile. It would appear that within the same profile, all the Echoes share all of the connected devices among them. When using a second profile, nothing is shared with the first profile.
Finding connected devices is hit or miss. I have 13 connected devices and Echo can find them all sometimes, and sometimes it would only find less than 10 of them. I don't know what causes this. And the only fix I know is to keep trying until Echo finds them all.