Seeing as an 8x or triple weakness to ghost/dark is impossible, as only psychic and ghost have those weaknesses, this is limited to the effects of Forest's curse.
8x to Ice
Dragon/Flying-Dragonite, Salamence, Rayquaza, Altaria, Noibat, Noivern
Ground/Flying- Gligar line, Landorus (both forms)
Dragon/Ground-Vibrava, Flygon, Gible line, Zygarde
8x to Fire
Bug/Steel- Scizor, Escavalier, Trash Wormadam, Genesect, Durant
8x to Bug
8x to Flying
And a triple weakness to Poison is impossible, as only two types, Fairy and Grass, are weak to poison.