I used to think MGallade should be S but that was before I realized how ridiculously influential Mega-Sableye is. MGallade doesn't need much support, but it definitely does need support to overcome MSableye teams since it can easily become a liability against them. The fact that Unaware is the preferred ability for Clefable at the moment is another draw against it. From what I can tell it still completely demolishes balanced/bulky teams which feature neither of these though. A+ is fine for it.
Mega-Scizor and Mega-Gardevoir are fine in A+ imo, MScizor is a great check to a lot of new megas (Altaria, Beedrill, Diancie, Metagross if defensive) and despite the popularity of Magnezone, it's still a really solid mega that can go both offensive and defensive quite effectively. MGarde doesn't suffer too much competition from MAltaria since they don't really fill the same roles : while MAltaria is a bulky glue/setup sweeper/wall, MGardevoir is just a nuke, and one which is really brutal for Stall and Balance to handle (which is pretty valuable right now given how good Stall is) and also has utility vs offense by being able to check a few things there such as Latis, Sceptile, Thundurus and Keldeo (usually scarfed nowadays which helps MGarde switch into it). Its effectiveness is probably due to the fact that people aren't really prepared for it though (most teams I see that aren't weak to it were built around MMetagross to begin with lol), once people start to take it into account more I could see it dropping since a couple of things (MMetagross and the increased popularity of Jirachi and Klefki) do hold it back, but right now it's very scary and probably still A+ worthy.