  An important global property of a bit string is the number of ones in it. It
has been found that the parity (odd or even) of this number can be found by a
sequence of deterministic, translational invariant cellular automata with
parallel update in succession for a total of O(N^2) time. In this paper, we
discover a way to check if this number is divisible by three using the same
kind of cellular automata in O(N^3) time. We also speculate that the method
described here could be generalized to check if it is divisible by four and
other positive integers.
