Answer
$Ar$ has the same electrons and the same electron configuration as $Sc^{3+}$.
Work Step by Step
$Sc^{3+}$
A neutral $Sc$ atom has 21 electrons. Losing 3 electrons to become $Sc^{3+}$ cation, it now has 18 electrons. The neutral atom that has 18 electrons is $Ar$.
The electron configuration of $Sc$ is $[Ar]3d^14s^2$. This shows that there 3 outermost electrons in orbitals $4s$ and $3d$ in a neutral $Sc$ atom. So, if 3 electrons are removed to make cation $Sc^{3+}$, these 3 outermost electrons would most likely the ones to be removed.
Then, the electron configuration of $Sc^{3+}$ is $[Ar]$, which is exactly the electron configuration of $Ar$.
