Despite the likes of Mega Charizard and Mega Mewtwo often stealing the headlines, there are actually far more gen 3 Mega Evolutions than there are gen 1. In fact, gen 3 Mega Evolutions make up almost half of the total Mega Evolutions in the entire Pokemon series; at least at the time of writing, anyway. While there are certainly some powerful Pokemon amongst this number though, quite a few of them are underwhelming to say the least.
Total base stats may play a big part in making a Pokemon great, but the way that those stats are distributed is also incredibly important. Without the right balance, a Pokemon might not survive long enough to fully utilize its biggest strengths or, conversely, it may lack the firepower to finish off its foe even if it's durable enough to last those few extra turns. With that in mind, the Hoenn region really does offer something of a mixed bag.
20 Mega Camerupt
While Mega Camerupt is incredibly strong in the special attack department and has a decent physical attack stat as well, its ridiculously low speed makes it a little difficult to use in competitive battles. It's all but guaranteed to act second and, with its defensive stats both around 100, is liable to being knocked out by a single weakness exploiting attack. If it does manage to survive though, it can at least deal some decent damage; especially when using moves with additional effects.
19 Mega Banette
Mega Banette doesn't quite fit the description of a glass cannon, but it's not too far off. Its 165 physical attack stat has the potential to be devastating, but whether or not that potential is ever fully realized will likely come down to the opposing Pokemon's speed. 75 isn't terrible, but given that most Pokemon used competitively have speed stats of 100 or higher, Mega Banette is going to be acting second more often than not.
18 Mega Altaria
Mega Altaria is an incredibly well balanced Pokemon in every department except speed. Sadly, in a world in which most of the top tier Pokemon tend to excel in either defense or offence, it's difficult to see where Mega Altaria really fits in. In all likelihood, it's either going to lack the firepower needed to go blow for blow over the course of a long battle or get instantly knocked out by a Pokemon with a high special attack stat.
17 Mega Glalie
Unlike Mega Altaria, Mega Glalie at least has its strengths. These come in the form of its high special and physical attack stats and are balanced out by some fairly weak defenses. Ultimately though, it doesn't quite lean far enough into this division to really stand out.
When all's said and done, Mega Glalie just doesn't have enough firepower to be able to reliably take out its foes in a single hit, but then nor is it likely to be defeated by one either. As a result, it's a fairly average Mega Evolution.
16 Mega Sableye
With just 20 speed, Mega Sableye is the joint slowest of all of the gen 3 Mega Evolutions and is also tied for the lowest base stat total. Despite its slow speed, however, it boasts a fairly solid defense and its Mega Bounce ability can be incredibly useful for dealing with entry hazards too. When paired with Recover it can be a useful staller, but due to its lackluster offense, Mega Sableye can sometimes lack the bite to deal with bulkier teams.
15 Mega Sharpedo
A 105 speed stat may not seem that great, but thanks to Sharpedo's Speed Boost ability, Mega Sharpedo is faster than most Pokemon by the time it enters the fray. This bodes well, as its poor defenses make it vulnerable to being knocked out by a single blow. It makes up for this somewhat though with its 140 physical attack stat and its Strong Jaw ability, both of which help it to deliver one hit KOs with Crunch fairly consistently.
14 Mega Absol
Mega Absol has a fairly similar build to Mega Sharpedo, although boasts higher speed and a higher physical attack stat. This does come at the cost of defense, but assuming it acts first and is able to attack its enemy's weakness, there's a very good chance that Mega Absol will be coming out on top. If that first hit isn't a knockout though, it may not survive for too long.
13 Mega Mawile
With its low base stats and abysmal speed, Mega Mawile really doesn't have too much going for it at first glance. Thanks to its Huge Power ability though, it effectively has an attack stat above 200; allowing it to dish out a lot of damage when given the chance. Sucker Punch can somewhat make up for its slow speed too, but this strategy can leave the Pokemon vulnerable to revenge kills.
12 Mega Medicham
Mega Medicham's 510 base stats may not seem all that impressive, but its Pure Power ability more than makes up for this. Like Huge Power, Pure Power doubles a Pokemon's attack which makes Mega Medicham incredibly deadly in certain situations. Its 85 defenses can leave it a little vulnerable, but its 100 speed and access to Fake Out help to mitigate this somewhat.
11 Mega Gardevoir
Mega Gardevoir has an incredible 165 special attack stat and backs this up with solid special defense. In the right situations, it can therefore be incredibly effective and should have the beating of most Pokemon that rely on special attacks and physical defense. It also boasts a strong learnset that includes many of the best psychic and fairy-type moves; the overwhelming majority of which just so happen to be classified as special attacks.