Why can we not use a 3 x Apple watch setup which are available used for 300 USD on eBay for the patients in the hospital?

In Rob ter Horsts tests, the Apple what is closest to his Polar H10 chest strap: But for other activities than intense activities with strong body movements, 3 channel ECG devices are gold standard. But can we not choose a setup with 3 Apple watches do have a gold standard as well? If we look …