Evaluation subtle differences in acidity?

I have be given that ethyne(acetylene) is a stronger acid than ammonia and ammonia is a stronger acid than ethylene. I am looking for a good explanation as to why? What I know is that ethyene(sp triple) has more s character than ethylene and that makes its conjugate base more stable than ethylene(sp2 double bond) but why is ammonia in the middle between the two with is sp3 hybridization?