Why are the bond angles represented below for each molecule not the same? they have the same form: one central atom and two others in the sides, so the bond angle has to be the same for the three, doesn't it?
Why should the bond angles be the same? You have different atoms every time leading to different molecules. Different molecules have different properties; that goes both for macroscopic properties (they will have different melting and boiling points, their liquid phases will have different densities and refraction indices) and microscopic ones (i.e. their molecular structure will be different). Indeed, there are three-atom molecules with a $90^\circ$ bond angle and those with $180^\circ$. I’m not sure whether it ever gets any smaller than $90^\circ$ but I suspect ‘no.’