I haven't really played much XY, but I feel like I have played enough against these Pokemon to at least reach a judgement on whether they warrant a ban. Mega Lucario is very reminiscent of Landorus-I in B/W where it has very few, arguably any decent counters, and to reliably stand a chance against it you have to run some obscure counter such as Zapdos or whatever. While running Zapdos may help you beat Mega Lucario consistently, the fact that you should have to run Zapdos on the odd chance your opponent is using Mega Lucario speaks volumes of the influence Mega Lucario has on the metagame and the impact it can have on teambuilding in general. If you are running a fully dedicated Mega Lucario counter, it's more often than not a liability and serves little purpose when your opponent does not bring Mega Lucario. It's also important to note that Lucario can run Special and Physical sets, adding to the unpredictability of Lucario and changing the definition of what a Lucario counter is altogether. Do not use revenge killing as an argument to deal with Mega Lucario - while it's certainly possible to deal with Lucario through a Scarf user or a Mach Punch, to get the opportunity to do so, you need a free turn, often meaning sacking one Pokemon entirely just to get the opportunity to revenge and even then there is nothing stopping the Lucario user from switching out again.
I'm also on the pro-ban side for Genesect, mainly for a lot of the reasons it was banned in B/W. Genesect is probably one of the most threatening Pokemon in the metagame right now, primarily for the fact that it can run so many different sets, all of which are very viable. Genesect is in the same boat as Mega Lucario in that it has very few counters, however all of Genesect's main counters are slower than it, so there is nothing to stop it U-Turning out. The same argument cannot be made for other users of U-Turn / Volt Switch, as no other user of these moves imposes the same threat that Genesect does and they are often slower than their counters [Scizor slower than Heatran, Rotom-W slower than Latias]. It doesn't even matter if you can get your Heatran in on Genesect, because it's going to U-Turn out into something equally threatening that can beat Heatran, or whatever counter you try to argue beats Genesect. This is why Genesect is so viable on offensive teams and warrants a ban as it is literally uncounterable. The prevalence of Defog also means that Genesect isn't being worn down every time it switches in, and in general dealing with Genesect is struggle as to fully deal with it you need to know the full set, by which time it'll probably have significantly dented whatever it needs to. When you factor in Download and Genesect's ridiculous movepool, it's easy to see why Genesect is so hard to deal with and for these reasons I feel a ban is justified.
I feel different about banning Deoxys-S, as I feel it has much less influence on the current metagame than the other two. As a hazard lead, Deoxys is reliable in setting up hazards, but not so much in making sure those hazards stay on the field for the entirety of the match - Defog users are incredibly common and Deoxys can be a liability sometimes against teams with Defog users. It doesn't have the bulk to continually switch in and setup hazards against more defensive teams, while offensive teams can OHKO / 2HKO it, often limiting Deoxys to 2 moves at best, which isn't going to guarantee you all the hazards you ideally want. The offensive set I have found equally underwhelming, as Deoxys does not have the best offensive capabilities, and is often prone to being used as setup fodder after a Psycho Boost which is detrimental for Hyper Offense teams.
If the objective here is to achieve a balanced and competitive metagame, then I feel banning Genesect and Lucarionite will help to reach this ideal metagame.
