Juniors have a mandatory 2A, a mandatory solo triple jump (this year its the lutz), and then a combination. Because of this, the only place a triple axel is allowed is in the combination, which is why you see Sofia Akatieva, Sofia Samodelkina, and Alysia Liu doing 3A+3T combos in the sp instead of just having it as the axel jump. However, not only is it more difficult to do the 3A as a combo, it also doesn't gain you as many points as it does in seniors. This is because when you do it in the combo, you're replacing usually a Flip (this year) or Lutz (other years), as opposed to replacing a 2A when you do it as the axel jump. Someone said it above, but doing this only gets you a couple points, which can easily be knocked out by a lower GOE if you have a slight stumble. This is one of the reasons Alysa Liu wasn't dominating the SP last year, despite being the only one in the junior field to have a 3A. I think this rule is stupid as so many of the girls have 3As now. Hopefully it changes along with the senior ladies quad rule, so that mens and ladies fields have the same rules.
So no they actually can do the 3A in the sp, its just more risk for less reward which is probably why Zhilina didn't do it, as she hasn't been doing her 3A for as long as Akatieva and her 3A+3T combo didn't look quite ready in the videos we saw. But I'm sure the ultimate goal is for her to do that this season. However, it was just recently that Zhilina was struggling with normal 3+3s, so its amazing that she's come this far at all.
They had the same layout pretty much, with the only difference being that Kamila had a stepout on one of her 4Ts. I thought they both skated really well that competition. However, they won't compete against each other internationally for another three years or so, so if there is a rivalry it will be a long time from now.