All relevant rewards (Shiny Charm and diploma) are obtainable without capturing Mythical Pokemon. If these are what you're going for, there's no need to get Diancie, Hoopa, and Volcanion. If you're looking to fully complete the Pokedex, however, then you will of course need to go for the Mythicals -- they still have Pokedex entries.
Source: Shiny Charm (Bulbapedia) and experience
