(sorry if this has been discussed elsewhere, but I couldn't find a thread dedicated to this)
So from what I remember, it was mentioned by Christopher in a podcast interview that 3 heroes die in the main Sentinels timeline. With the reveal of Iron Legacy as a villain, it was confirmed that Legacy was one of those heroes. I was surprised by this, but in hindsight I feel like I should've seen this coming. It was right there in my face the entire time!
Well, right there on the table.
I think the incapacitated sides of heroes are not an "if" but a "when" situation. Most show the heroes beat up, but a couple do show them... worse. Legacy's is an homage of the Death of Superman, so it totally makes sense that he dies in the storyline (Zalrus9 also pointed this out at one point). Young Legacy's also shows her dead, but that's been revealed to be the catalyst for the dark, alternate future where Iron Legacy rules.
So then who are the other two? Well, going by incapacitated sides, I'm going to guess Mister Fixer and Argent Adept.
F6 Unity is wearing Fixer's hat, indicating that he's gone. Well, his flipped side is nothing but his hat left, and that makes me wonder. Granted no body is seen, but my gut tells me the old guy doesn't triumph over the Chairman.
As for the Adept, his incapacitated side is an homage to an Uncanny X-Men cover where Wolverine is in the same position. Wolverine almost died in that situation, and Jubilee saving him is in part why he survived. Thing is... Logan has an amazing healing factor which helped him survive. I don't think Anthony Drake has one of those, so his chances of surviving a similar situation seem bleak.
But that's just me speculating. An argument could also be made for Fanatic, but I just get a feeling she's tougher than that.