Single lead, 2-lead, 3 lead ECG devices, which one to choose to validate HRMs for sporting activities and clinical applications?

The definition of 1,2,3 lead ECG devices are not always easy to understand for non-cardiologists as we are. If we have understood it right, a 3 lead-ECG device has 3 channels which are amplified and analyzed independently from each other. https://www.ebay.com/itm/303453638578?hash=item46a73f0fb2:g:KjYAAOSwNw1eJUjd&amdata=enc%3AAQAHAAAA4H7s7GvIQeeSM96MzUrqIZ%2BdQ%2B93E8ETSp9T6pYnMXpWQ934aPhgMKmiNukDkP5rcXhsu0tKX8M8RhiEQkx%2F%2FT033%2FlVpWnMrTOxZ668rfr7fcihZjChMbX6k5KQpaPtWJDkcVjs2WtmC5TCyE5OuWDF3vBeVNIuSsw2NgSj67B0uPPSfTgVKh1U36O%2B5eHs2qp2P%2By7txGPcq9JOJKBMWSVfcvvMD1PKg%2B2ta4COELlbeQyE4p0fRW1MXCyQQ1lI5DODM%2BVYI5aG4mWxwKXrW0epI5AidDG6yPhWEfbOu4E%7Ctkp%3ABk9SR660sM6oYQ Validation papers for sports sensors with ECG devices as a reference device, in which …