No, 3 million electric toothbrushes were not used in a DDoS attack::A widely reported story that 3 million electric toothbrushes were hacked with malware to conduct distributed denial of service (DDoS) attacks is likely a hypothetical scenario instead of an actual attack.
If its the same one my neighbour has, your neighbours can also track your brushing habits. My homeassistant instance automatically picked up my neighbours oral-b toothbrush :o