Ways to differentiate three respiratory signals
You're talking about ECG-derived respiratory (EDR) signals, but you didn't detail how you obtained them. Please detail your methods. Maybe then it will be easier to understand what each of your signals mean.

By 'experimental signal', I take it that you mean the reference/gold standard annotation of when respiration occurs, am I correct? But you did not detail if this was the onset or end or peak of the breathing cycle, as well as what kind of measure it was (tidal volume, airflow, other measure?)

Edit #1:
As for features, frequency is a good starting point. EDR signals may not evolve the same way as the reference signals but they should evolve at about the same period. If the breathing is controlled during a significant amount of samples, you can compare the frequency of the whole signals. If not controlled, then take segments of more than 2 seconds. You can apply FFT to get the frequency.

Phase will also be of interest, but it will difficult to comment further without knowing the algorithm for calculation of the EDR as well as the reference signal.