Which actinoid from following has smallest ionic size in $+3$ state?

In the actinoid series, as the atomic number increases from Actinium (Ac) to Lawrencium (Lr), there is a gradual decrease in the size of atoms or $M^{3+}$ ions. This phenomenon is known as actinoid contraction, which is due to the poor shielding effect of the $5f$ electrons.

The given elements belong to the actinoid series with the following atomic numbers:
Pa ($Z = 91$)
Cm ($Z = 96$)
Fm ($Z = 100$)
Lr ($Z = 103$)

Since Lawrencium (Lr) has the highest atomic number among the given options, it experiences the maximum actinoid contraction. Therefore, $Lr^{3+}$ has the smallest ionic size.

Answer: Lr