5-B to 5-A looks much better than 3-A,
This destruction is said to destroy the dimensional worlds, each dimensional world contains a lot of worlds, even space between them (the destruction is directly dimensional, so it has to overcome the distance between the space). This could even be more than large planet level, if there is any sweep of the distance between them.
If we know the range of interdimensional distance from at least any scans or expression, it really needs to come out more from the large planet level.(if you have any, you can throw)
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However, I cannot say that I totally dismiss the 3-A issue because you said that you simply said that universes can contains, and there is a multiverse in the series. But it is said that there are countless Earths in scans alone. Yet, that's not why I didn't reject the 3-A thing, but the main reason is, If I look at it from a slightly different point of view;
As I understand it, the dimensional world is the uppermost structure that contains all the worlds. Isn't the place where Nanoha lives is an alternate universe within the multiverse? this should mean that the dimensional world referred to as the uppermost structure is actually an alternative universe. yes it is not infinite, but it goes by that the navigation is slower than the transporter, it is said that dimensional transfer cannot cross very long distances. Since it is mentioned that this explosion will destroy the dimensional worlds, so I cannot reject the 3-A part. (but this is also a very different point of view)