An ion M³⁺ of the 3d series has the configuration 3d³…

The d block is groups 3 to 12. A transition element has an incomplete d subshell, with 1 to 9 electrons, in its atom or in a common ion. The 3d series fills $3d$ after $4s$, except Cr ($3d^5 4s^1$) and Cu ($3d^{10} 4s^1$). An ion loses its $4s$ electrons first, so an ion $\text{M}^{q+}$ has $Z - 18 - q$ electrons in $3d$ once both $4s$ electrons are gone. By Hund's rule, a subshell with $d$ electrons has $d$ unpaired electrons if $d \le 5$ and $10 - d$ if $d > 5$.