Phoenix RISING
Banned
You can get one of them, but all at once (+mercy. With her current rez, she's part of the cancer comp) requires coordination (though Sombra helps out the most). Which is why it's problematic. It's easy as pie for the defenders to use, they just need to pick these 3 characters and natural play takes care of the rest. The attackers actually have to plan out who goes after who, when Ults are gonna be available, etc.
Indeed, it is a team-based game, so yes, you will have to depend on coordinating with your teammates to combat a coordinated team.
If you can take out one--Mercy would be ideal, but taking down Torb and/or his turret is comparably efficient--then it's up to your teammates to do their jobs. Even just making sure that Torb never gets his turret back up to level II is enough to get a push going.
Dva, as my 8 year-old son who is on the precipice of platinum on PS4 has demonstrated, is an incredibly strong deterrent to all three of those heroes, especially with her missiles.
I feel like a "meta" that includes static defense like Bastion and Torb on the same team is a result of folks not thinking outside of the box enough. There's probably some hero that everyone thinks is bad who is the answer, lol.