I like Stella and Julien together. Scott kind of sticks out - it’s a nice name just kind of bland in comparison to the other two. I think Sebastian would fit perfectly. But I’m not into super matchy sibsets anyway so I think Scott is fine to use!
Firstly, I go out of my way to make sure, even in hypothetical situations, siblings get different first initials. Secondly, Stella sounds so much older than Scott and Julien. I can't imagine Stella as a child at all.
It's okay, but I don't think Scott goes with Stella and Julien. Scott sounds like an old man's name to me, while Stella and Julien feel modern and youthful.