Did something come out to say that the Box Theory was debunked? Why do you think there are 3?
There's no confirmation that there are 3 newcomers remaining, but there are several factors that make me believe that's the case:
-The Box could be incomplete in order to prevent leaks. If true, then we might get Ken+1 other newcomer (probably Incineroar) in October, and more characters in November.
-Isabelle was announced at about the halfway point in the hype cycle, if we consider E3 the start of the cycle and the release of the game the end. Before her, there were 3 newcomers announced (Inkling excluded because they were announced before E3), so it would make sense if there were 3 left. Not counting echoes here.
-In the August Smash Direct, they announced 2 newcomers, and so have set our expectations for smash directs. I am expecting another smash direct in November, and I'd be surprised if they don't give us another 2 newcomers because they are fully aware any less would disappoint people.
-If the corocoro 108 stage leak is true, it increases the likelihood that there are characters remaining from unrepresented franchises, which will come with a new stage. We only have 2 new franchises introduced so far, which would be the lowest in series history (Melee introduced 3).
-I find it very unlikely that they would stop at 69 characters (echoes excluded) and not try to hit the 70 benchmark.
-The characters on the site are arranged in rows of 3. This could be intentional so that when all characters are revealed, there are no empty spaces. If true, that means that including echoes, there must be either 2, 5, or 8 characters left. Since Ken is pretty much already confirmed by the box, that means we'd only get 1 newcomer and I've already explained why I don't believe that's true. 8 seems too optimistic unless there are a ton of echoes left. 5 fits with what I've been thinking perfectly, it'd leave room for 3 more newcomers and 2 more echoes.
Not saying any of this is confirmed or even good evidence, it's just my scattered thoughts on what might be left.