As far as the odds of finishing, assume each hiker has a one in five or 20% chance of finishing. The chance all three finish is 0.2 x 0.2 x 0.2 = 0.008, less than 1%.
Of course you guys are young but there are other complications such as two being siblings and all of you knowing each other to start with. It's hard enough hiking the entire trail without having to adjust what you're doing for other people. If someone has to lay out for two weeks, what do the others do? Make sure that you have independent gear kits, that you are sharing a tent or cooking pot.
Make a rule that no matter what kind of a mood you're in or despite any disagreements, nobody goes to sleep at night angry, that things get resolved before you go to sleep. That would significantly increase your chances.
The camera guy is not thru-hiking but that puts a whole 'nother twist on it.
Not trying to discourage you and it is just walking up and down mountains all day. But sometimes that just isn't easy.