3sec power is actually a one sec power?

The 3s, 5s, 10s etc. averaging is done in the head unit using the raw data from the pedals.

I know that.
The matter is that all the above mentioned PMs worked exactly as you said. But when I switched to Rally pedals the readings (averaging) changed. Could it be the Rally SW bug?

The Rally manual lists all the data fields for the Edge 1030.

Michael_Kalina said:

I have 3sec power field only on my Edge and when I do a sprint I see readings like 1100-1200watts several times but I get 5sec power like 840watts.

It also makes sense that your 3s power is greater than your 5s power because the peaks are less smooth.

A possible way to test whether there is a bug with the pedal data might be to set up a page on your Edge that shows each of the power fields ie current power, 3s 10, 30s to see whether they differ from each other during a ride.  If they are the same then there's possibly a problem.  Even the 1s value (if it is indeed 1s) within the pedals will be an average of all the measurements throughout that second and that means there will be some filtering/smoothing in the pedals.  The filter will not be identical between different manufacturers so I would expect some (small) variations in current power readings.