Alright, I see three branches forming then (P1&3, P2&3, P2). You can either manage it manually (by writing three separate story tracks), or by using variable scripting or on page scripting (e.g. show some text only if person 3 is alive or/and if person 1 or 2 is with you). I'd recommend you start out with the multiple manually written paths approach for now (it's easier to proofread).
If you're trying to figure out how to randomize person 3's death, you can use Link variables. Link variables mean that when a person clicks a link (in this case chooses path 2), something happens as is noted (in a variable). You'd need to use the RESET command. Say the person has a 40% chance of dying, have the Link script be RESET 1-10 on a variable (say Person 3). Then, on the next story page, have two links - one that only shows if Person 3 variable is >4 (i.e. he's alive), the other only when the variable is <5 (i.e. he's dead). If this sounds confusing, roll with the first paragraph.