Three ECG probes are placed on 100 patients to monitor their heart pulse. Pulses from these probes are used to automatically diagnose the patients as having arrythmia or not. Each pulse is recorded for a duration of 1 minute at a sampling rate of twice per second. A many-to-one RNN is trained on the pulses to make arrythmia diagnoses and it achieves an accuracy of 95 percent. How many sensing neurons did this RNN have?