^Probably the same way they did between 1 & 2. Really, none of your decisions in the first game had much of an impact on events of the second. It just decided what characters would or would not show up depending on who you let die or killed. If they're dead then they either didn't show up at all or were replaced in their role by another character. Even saving or killing the council had little effect on the plot.
Don't get me wrong, it makes a huge difference to one's individual gaming experience and immersive as hell to see the consequences of your actions if you imported an old save, but nevertheless, it's really little more than window dressing.
I'd say the fact that you can in theory have all but one of your team killed off by the end suggests that few, if any of them will have any pivotal role in the third instalment, or if they do there's the option for the writers to replace them. Another Legion platform if your one didn't make it or Thane's son if Thane himself was killed, etc.
Personally I'm hoping Wrex, Ash and Liara all come back together at some point so you can have a proper reunion.