Stunfisk will undoubtedly be the hardest to make viable. It has no real niche, disappointing stats and movepool, and a rather useless selection of abilities. Its one redeeming quality is its decent-at-best 109/99 special bulk, slightly better than its physical bulk. However, a Ground/Electric typing, albeit interesting, is not amazing defensively, giving Stunfisk weaknesses to common moves like Earthquake and Scald. (Notably, it synergizes almost perfectly with pure Fairies, resisting both Poison- and Steel-type attacks and hitting the Fire-, Poison-, and Steel-types that give Fairies trouble super-effectively with Earth Power or Earthquake.) There isn't really any feasible way of making Stunfisk an offensive threat without giving it a +50/+50 stat renovation and an overpowered ability, so I think the best way to go about Mega Stunfisk to turn it into a mixed wall. 109 is a pretty good HP stat, so adding 20-30 points onto its middling defenses can make it harder to break. It doesn't have a fantastic support movepool, with its options being limited to Thunder Wave, Toxic, Sandstorm and Yawn, but most of its attacks have dangerous side-effects, like Scald, Discharge, and Earth Power (10% chance of lowering SpD). This screams Serene Grace, which, flavor irony aside, would be a great fit for Mega Stunfisk. This probably requires a good-sized boost in SpA if it wants to do any damage while it burns and paralyzes stuff. But before this ugly little mud fish turns into an annoying piece of shit, it still has two gaping flaws if it wants to be a successful wall - it has no reliable recovery, and it is hit by all entry hazards and status effects (except for Paralysis, which, let's be honest, Stunfisk wouldn't give a damn about anyway). Serene Grace also makes little sense flavor-wise, so I perused Stunfisk's Bulbapedia entry, and I found that it protects itself so well it should be a celebrity spokesperson for Trojan: "Its skin is very hard, so it is unhurt even if stepped on by sumo wrestlers...it conceals itself in the mud of the seashore...when prey touch it, it delivers a jolt of electricity". An ability reflecting its uncanny capacity to guard itself from others would make perfect sense. Magic Bounce is the first thing that came to mind, but it doesn't nullify entry hazard damage, and Magic Bounce users are still susceptible to statuses as side effects from moves like Scald, so I went with Magic Guard. MG users are basically immune to any types of residual damage, whether it be from Spikes or poison, so can really only be damaged using direct attacks, most of which Mega Stunfisk will hopefully be able to take with ease and make it much harder to take down. I'm somewhat stuck between the two right now, but regardless of ability I'm probably going to add some form of reliable recovery, the lack of which is the other main flaw I noticed. I'll probably also keep the Ground/Electric typing, but I'm not ruling out removing one of those.
Accelgor is mainly used for its lightning-fast support capabilities and spike-stacking, and as a bug type I can't resist giving it sticky web. However, if it wants to set up a more than one layer of Spikes as well as SW, it really needs better defensive stats. If used as a lead, it can pretty much outspeed anything, so we can probably give it an ability besides Unburden, although I'm not sure what. I was thinking of giving it something like Oblivious, changing its typing to Bug/Poison and giving it Toxic Spikes to make it something of an ultimate hazard/support lead, but that's one of many viable options. Its Special offense could use a boost both stat- and coverage-wise, although it usually doesn't have more than one or two moveslots available for actual attacks, but the real problem is its physical defense - 80/40 is absolutely pitiful, so a +40/+50 boost might be in store for this little ninja bug dude.
These are just some preliminary thoughts, and I've obviously examined Stunfisk much more thoroughly as of now. I'm also thinking about the other two, but I'm having trouble finding a different role for their mega to fill, especially with Mienshao. I can always just boost their highest stats and call it a day as a last resort, but that's no fun and it doesn't encourage creativity, which is the whole idea here.
(also this is my first post here after lurking for a month hooray so hopefully I didn't do anything wrong aha)