Malphite, not being a fanboy or a devil's advocate here, has always been the anti AD guy, along with Rammus and Taric. Malphite thrives in triple AD comps, which are quite common now due to strong AD tops like GP, Fiora, Graves, and others along with junglers like Graves and Kindred. Plus general consensus among the community is AP mids are weaker than the AD ones, which I don't fully get but that's just me. And of course we always have ADCs who use AD, Corki and Ezreal being the oddballs.
In addition, Malphite as a whole is just quite reliable with a massive knock up functioning as a gap closer to boot that is devastating in team fights. He can basically function as an Ult bot and win games because of the level of impact he offers. Plus his laning phase is reliable as well due to GOTU coupled with his passive and decent trades vs. most top laners.
I can't speak much for Ahri; I haven't played her in forever. I would think it has to do with her just being highly reliable and favorable in most match ups. Ahri doesn't really have any match ups that are just abysmally horrible. Like, TF into Fizz horrible or Jax into Teemo horrible or something super not fun.
Graves...Idk man. Graves is just so weird right now; functioning super well in 2 roles, arguably 3, and 2 of those aren't even his intended role. It's just the backlash from the update he had and experimentation that followed suit. I mean, his kit is amazing for jungling, just amazing. Only problem is if you go too hard on Graves, it can cost him late game, which is his entire purpose on the team. The Q nerf was fair as it gives more time to react in the corridors to potential damage considering how fast his Q fires and that he is most likely near a wall anyway. I could see an ult CDR nerf at lower levels. Maybe even some reductions to his True Grit bonuses. Time will tell.