A d-dimensional polycube of size n is a connected set of n cubes in d dimensions, where connectivity is through (d-1)-dimensional faces. Enumeration of polycubes, and, in particular, specific types of polycubes, as well as computing the asymptotic growth rate of polycubes, is a popular problem in combinatorics and discrete geometry. This is also an important tool in statistical physics for computations and analysis of percolation processes and collapse of branched polymers. A polycube is said to be proper in d dimensions if the convex hull of the centers of its cubes is d-dimensional. In this paper we prove that the number of polycubes of size n that are proper in n-3 dimensions is 2^n-6 n^n-7 (n-3) (12n⁵ - 104n⁴ + 360n³ - 679n² + 1122n - 1560) / 3.