Only thing I see being used by the top players re always the same team (i.e. garchomp, mega lang, room-w, talonflame).
I wished there was something nintendo could do to make the 3v3 even remotely not-too centralizing.
I cannot find the tentative XY OU tier list (if anyone could provide that link to me that would be awesome) but my guess is that most of these Pokémon you've mentioned are within the top 5 of OU (except Mega Kangaskhan, who let's face it would be top 5 for sure if it wasn't banned).
Because 6v6 and Battle Spot play so differently, I think it makes complete sense that these Pokémon would find their way onto the majority of teams. I mean, if someone was making a 6v6 OU team and simply used the top 6 Pokémon of the OU tier, that team probably wouldn't work out very well for them. But when using half that number of Pokémon, it suddenly becomes more feasible to play that way.
But yes, I agree, it does suck a bit that these are some of the Pokémon you will see or have to use in order to get results. I'm not saying the ONLY way to win is by using these Pokémon, but more often than not, they will win you games.
However, I believe that this can open up the door for unique and out-of-the-box ways to try and play in Battle Spot. For example, I've been using SubDisable Gengar on my Battle Spot team. For one thing, people expect you to be using Mega Gengar, so they don't usually see it coming. For another, it checks a fair good number of stuff. Choice users have always been checked pretty well by SubDisable Gengar, but now that your team is only half of what it is in 6v6, the momentum gained by Gengar is drastic. Another thing I find SubDisable Gengar checks is Mega Kangaskhan, actually. In my experience, people will Mega Evolve Kangaskhan as soon as they can, so 75% of her moves won't even hit Gengar. There's Crunch, but looking at the Battle Spot Season 3 stats, Crunch isn't used on Mega Kangaskhan all that often. So they try to Sucker Punch you, and you just sub down. Same for Mega Mawile. I know SubDisable Gengar isn't exactly "thinking outside of the box" because it's been done before, but people just aren't expecting to see it in Battle Spot, I guess.