I think its really controversial
If you want to simply out level foes and beat them easily, blaziken is more then enough, as there are alot of foe weak to fire/fight/both, and almost no foe has resist to both fire/fight in one mon, also you can use blaziken to beat 2 of 5 elite 4 as well with super effective move.
Considering grass not effective in typing (sceptile), water is very common in RSE with many water choice better, which you cant have super effective (swampert), and also flying (sceptile problem, swampert ground move no efffect), blaziken is better in view less damage to it.
Ice water combo is common as well, and you got alot of fight with water flying ice and alot on water route, meaning blaziken disadvantageous to be not in full hp or not effective, if you want safe, swampert is nice to resist ice water combo, which in exchange you wont have any nice fire type except trading in a charizard ot something.
So just my view, from disadvantageous side, you shall consider what you dont mind and go ahead with that
If you dont mind grass type killing you flying type troubles you (which elite 4 no 4 is a big trouble), go swampert.
If you dont mind grass type has mo strong move like fire type, go sceptile
If you dont mind water and ice issue, or you want strong movesets like flare blitz and overheat and so, go blaziken
I think got alot people teaching u from nature/iv/ev/movepool/typing/damage etc, you consider your choice and play okie, its just game, you can slowly play and out level and kill in less effective situation...... For example going elite 4 in lv 70++80++ sure you kill alot and pass easily..
If you simply cant make decision, go with all three, as you can trade as early as after you go to the first pokemon centre in RSE.
I had once played with all three starters by trading before first gym.
When beating elite 4, blaziken lv 63, swanpert level 40, sceptile only 32...