Amplitude calculation
You have to be specific about your selection criteria. All you're "big amplitude" are local maxima but there are also other local maxima that don't qualify (why?). It looks like you want a local maximum that's above a certain threshold. Or maybe you only want the top 3? 
Anyway, you can find local maxima by either differentiating and finding the zeros or my a simple search by looking at the neighbors of each sample. Then this can be combined with a threshold detector or a sorting algorithm.