Mega Gallade
Typing: Psychic/Fighting ----> Psychic/Fighting
Abilities: Steadfast/Justified ----> Knight’s Honor (when stat drops, Atk and Speed are raised by 1 stage each)
BST: 68/125/65/65/115/80 ----> 68/150/110/70/135/85
New Moves: Sacred Sword
If Bisharp looks like a samurai, then Gallade certainly looks like a knight. The ability works somewhat like Bisharp's Defiant, but since Mega Gallade doesn't have a strong priority move, the buff becomes +1 to both Attack and Speed instead of +2 to Attack. The added bulk helps with Mega Gallade's chance at setting up either Bulk Up or Swords Dance, and Sacred Sword, of course, is to provide a reliable STAB other than Close Combat which makes Gallade frailer in exchange for more power.
Mega Probopass
Typing: Rock/Steel ----> Rock/Electric
Abilities: Sturdy/Magnet Pull/Sand Force ----> Levitate
BST: 60/55/145/75/150/40 ----> 60/60/160/125/170/50
New Moves: Focus Blast, Calm Mind, Automize
Mega Mantine kind of called it.
Since there's already a defensive Rock/Steel Mega a la Bastiodon, and Rock/Steel STAB kind of sucks, I went with the Rock/Electric typing for better STAB. Levitate removes Mega Probopass' crippling 4x weakness to Ground, and the decent bulk allows it to set up Calm Mind or Automize. Problem is, Probopass is as slow as Aegislash, yet does not possess any priority, which means it'll have to take hits often. It also has absolutely no recovery, which means it can be whittled down.
Mega Froslass
Typing: Ice/Ghost ----> Ice/Ghost
Abilities: Snow Cloak/Cursed Body ----> Competitive
BST: 70/80/70/80/70/110 ---> 70/85/95/129/90/111
New Moves: Focus Blast, Nasty Plot
Froslass' BST is lower than I expected, so this took quite a bit of work. With Competitive, Mega Froslass becomes another great switch-in to Defoggers; barring Crobat and Scizor, a fully invested Timid Mega Froslass at +2 Sp. Atk thanks to Competitive outspeeds and OHKOes every single common defogger with either Ice Beam, Shadow Ball or Focus Blast. Okay, except for Mew, but that's a 56.3% to OHKO as well. The 111 speed is to outspeed fully invested Latias and Latios, and 95 Def allows it to have a 62.5% chance to live an unboosted Bullet Punch from Mega Scizor with a 0 HP / 4 Def spread. Even without an opposing Defogger, Mega Froslass can set up Nasty Plot by itself, though it'd be harder due to its medicore bulk.