BECAUSE SENTINELS ARE SHIT- (The role, not the team. They are too but that's a topic for another day...)
Sentis have been nerfed into the ground and get overrun by duelists, you already want to run 2 duelists in todays meta, what's so bad about 3?
Duelists like Neon/Phoenix give the same utility as a support initiator while also being able to play a lot more selfishly when they have to. It's the whole reason people like Erde/Jamppi play phoenix and Ethan played Neon, Those agents are initiators with entry tools. You get two of your best players on a role they're comfortable with, and get a 3rd for extra utility. While you could run Kay/o or Breach for a similar effect, why would you do that? Those two are also really bad right now. Breach's flashes/stun are easy to dodge and Kay/o is just worse than Sova/fade aside from a single map which you would also run Sova/Fade on which is ascent...but His job got stolen by phoenix. So, if you want extra initiator utility without sacrificing much when it comes to comp lethality...why not run a 3rd duelist?
3 duelists is also great for faking sites, waylay can dash in to show presence and then get out while the rest of the team scales the other side of the map. You can risk losing one duelist since you have another 2 for site takes later. Duelists are stacked with so much utility that losing 1 doesn't mean sacrificing much of anything since you have 2 loaded kits left to take site.








