So, we’re in “3 dimensions” because there are only 3 directions, and every other direction is just a combination of those 3 (and their negatives). When string theorists talk about space having 10 dimensions they’re talking about all but four of those being “tiny loop dimensions”. The smaller dimensions are like that, but loop on a distance that is much, much smaller than the smallest particle. As a result, it doesn’t really make sense to talk about movement in these tiny dimensions, basically because there’s no place to go. In the loop dimensions there isn’t enough room, so as far as we (and even particles) are concerned, they may as well not be there.