What will you be using this phone for?
The a5 is the lowest end out the lot so it wont be able to do as much as the other 2 but should be enough for basic usage.
The note 5 is mainly for people who like using the phones as a mini pc as it can do a lot and it has a stylus which is good if you like drawing apps.
The s6 is more for people who want a powerful phone without the stylus functionality. Also I think it is lighter than the note 5.