For a B-tree of order \(m\), the maximum number of keys at height \(h\) is given by the formula:
\[
\text{Maximum Keys} = m^{h+1} - 1
\]
Substituting \(m = 5\) and \(h = 3\):
\[
\text{Maximum Keys} = 5^{3+1} - 1 = 5^4 - 1 = 625 - 1 = 624
\]
Thus, the maximum number of keys is 156.