After doing a lot more battles, I have decided (for now) to stick with my SpD Sableye. This thing... it has given me wins way more often than not. If I lose, it's because I was outplayed or I brought the wrong mons. If I win, it's often because of Sableye. Max SpD has saved me from specially oriented walls like Slowbro, and it can still take a few hits physically if the enemy is burned. Priority WoW is invaluable in this physically oriented metagame, and Priority Taunt has stopped all manner of enemy shenanigans, from Speed Boost/Double Team Baton Pass chains to SDs once they're burned, to prevent Aegislash from using King's Shield so my Talonflame can attack without fear, to random Furfrous (wtf why are people using him?), to freaking Klefki crap, the list goes on. Again, though the problems I have are what to use with it. I can't decide on a Mega to use on my team, and actually most of the time I barely even use my mega at all. I brought MMawile for a bit, and never used it. I brought Megazard X instead for a while, and barely used it except against a grass/steel heavy team or a mono Ghost team where it got me a win. Most of the time, I bring Gastrodon, Talonflame, or Tyranitar. Sableye disables a large part of the metagame that I feel it's better to go for a defensive lineup to out stall the enemy team, or to bring Talonflame in for priority clean up/or if I see Venusaur or other Grass types.
There's so much prediction involved in 3v3s, though, and I often end up mispredicting stuff because I think the opponent is predicting what I'm going to do. For example, MMawile gives me headaches. Do I burn it first turn, and then Taunt it but risk letting them get a SD, or do I Taunt it first to stop SD and then burn it but risk getting hit during the Taunt turn? That, and there's always a time where I know I could have won the match had I all 6 of my mons against their 6. I need to experiment more with different team lineups, but Sableye ain't going nowhere.
I might actually try using MBanette next. Priority Destiny Bond is nothing to sneeze at, and it'd make a great replacement if Sableye bites the dust. But then who to use with both of them...
Also, 252 Speed EV Talonflame bros. I've outsped and killed a decent amount of Talonflames with mine, it's hilarious. Another thing is I didn't know that if I defeat the opponent's last mon, but I die to recoil after, I still win. That's nice, considering that's what usually happens with my Talonflame...
Edit: To perhaps get some feedback, I'm currently running Cloyster/Sableye/Gastrodon/Tyranitar/Megazard X/Talonflame. Gonna put in Banette instead of perhaps Cloyster or Megazard X, I dunno...
I also feel like I should have a special attacker (Gastrodon doesn't count) but I have no idea what to use.