Same problem for me. The ECG function is close to useless, with exactly the same symptoms as described by the OP. Unless the pressure on the sensor is just perfect (not too much/too little) the trace occupies the entire height of the display, or the software keeps prompting me to touch the sensor.....while I am....
Remaining sufficiently still to avoid error messages is close to impossible.
For me too, this was the main reason for buying the Watch 3.
BP readings are sufficiently inaccurate to make them useless, SpO2 the same.
Heart rate monitoring (Continuous....lol) during normal exercise (e.g. walking) can be for as little as 10% of the time.
Getting ready to return this as "Not Fit For Purpose" unless there is a very positive response from Samsung.
There is an update available (just noticed it today) which I have installed.
The BP accuracy is no better - Systolic reading is frequently 30% off (generally too low), whilst Diastolic is +/- 10% off the true level.
ECG is still almost impossible to use, as the trace varies so much with the slightest change in finger pressure, and it is really difficult to be sufficiently static for 30 seconds.
So - SAMSUNG - how do I get this resolved? ECG and AF detection are the main reasons that I bought this watch.
Add to the above problems the significant HR drop-outs when walking, and I repeat - this device is not fit for purpose. Continuous HR monitoring simply is not provided, and the more important ECG/BP stuff is either too difficult to use reliably, or just too inaccurate.
I have just managed to perform a proper diagnostic test (found the "Test Mode" via the XDA Developer forum). There is a test which checks the ECG sensor and gives a numeric reading for the sensor (0 to 9), where 0 is fine, and anything over 5 is a faulty sensor.
See YouTube for a run through of the diagnostic tests
Mine scored a 9 on this ECG sensor test.
So, time to get this Watch3 replaced, I think...