How can I find the successive fluctuations of the ECG data.

Imagine I have an ECG data as such in the screenshot. Its just a random figure from the internet because I havent cleaned my ECG data.

I would like to find the successive fluctuations.Suppose that peak 1,2,3 are the successive R peaks. (largest peak on each ECG wave)

Fluctuation(n)=(Time at Peak 3- Time at Peak 2)- (Time at Peak 2- TIme at Peak 1)

I havent found a method to solve this. I was thinking of using findpeaks() and using MinPeakDistance since in ECG signals there are 3 peaks per wave. However, the heartbeat is very iiregular so this method doesnt seem to work. Perhaps, I should try FFT? Could someone show me their code and ideas for this?

Thank YOU!