Irregular waver form trendline fitting

I need to fit a trendline in terms of x of a function that looks like a log wave? Three points need to be set that determine what the wave looks like. The wave at x=0 gives an amplitude of zero, at x1, gives y1 at x2 gives y2=0 but x1<x2<x3 at x3=1 we get y3 which has a reverse Please look at the attached xlsx file