Does や still imply other elements can be added here?
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や is a parallel particle.
A, B and C have almost the same strength, all 3 are strong.
"Does や still imply other elements can be added here?" → Yes, kind of. When we say "BやC", we're not ruling out other elements for comparison. Because this type of comparison doesn't have to be so strict.
If we want to compare strictly A with B and C, it could be said for example "Aがいくら強いといってもBと比べて、またCと比べてそれほど変わるもんじゃないはずだ".