You hit a trick question. It does in fact end up as [Ar]3d3 when it becomes a 3+ cation. However, because it has half-filled stability, neutral chromium is said to have an electronic configuration of [Ar]4s13d5. This means that it loses 1 e- from the 4s and 2 e- from the 3d. This just happens to be one of the exceptions. Your answer is true for most transition metals that assume a 3+ charge.
