Mega Chesnaught
Typing: Grass/Fighting --> Grass
Ability: Overgrow/Bulletproof --> Bulletproof
Stats: 88/107/122/74/75/64 --> 88/127/147/84/100/84
Mega Chesnaught is now a pretty effective mixed tank. It has a lot of physical bulk, a good amount of special bulk with an ability that blocks some pretty common special attacks, and a good physical attack stat to top it all off. Its new typing turns its nasty 4x weakness to Flying and weak to the common fairy type for a much less common bug type weakness. It can run a effective subseed set and a pretty nice curse set. Mega Chesnaught is not really outclassed by Mega Venusaur, although it may look like it, but Mega Chesnaught has access to spikes, plenty of boosting moves, pain split, spiky sheild, and a higher defense stat.
Mega Greninja
Typing: Water/Dark --> Water/Fighting
Ability: Torrent/Protean --> Skill Link
Stats: 72/95/67/103/71/122 --> 72/135/82/123/88/142
New Moves: Arm Thrust, Bone Rush
Mega Greninja takes on the role that Normal Greninja usually neglects, namely as a physical attacker. It has a nice ability that gives it pretty powerful Water Shurikens, capable of having a total BP close to a Brave Bird from Talonflame without the recoil, a nice coverage move in Bone Rush, and another STAB option in Arm Thrust. It's high attacking stats means that it can run both a Mixed and a Physical set, although for special sets its outclassed by its base form. Mega Greninja has gained its new fighting type so it can obtain STAB on Arm Thrust and I always thought of a Ninja to be more of a Fighting type.
Mega Diggersby
Typing: Same
Ability: Pickup/Cheek Pouch/Huge Power --> Simple
Stats: 85/56/77/50/77/78 --> 85/106/92/60/92/88
Mega Diggersby is meant to be a powerful setup sweeper. While unfortunately, because of its new ability it is a bit weaker, but Simple also makes up for it by getting a very nice Swords Dance and Agility boost and give it the ability to be a very nice sweeper. Its new bulk lets it take some hits when setting up and with its new speed it can outspeed a nice amount of Pokemon. Mega Diggersby may not have Huge Power anymore, but do not make the mistake that its not a threat