Which of the following exhibits only $+3$ oxidation state?

Actinium ($\text{Ac}$) has the electronic configuration $[\text{Rn}]6d^{1}7s^2$. It loses all three valence electrons to achieve the stable noble gas configuration of Radon, thus exhibiting only the $+3$ oxidation state. Thorium ($\text{Th}$), Protactinium ($\text{Pa}$), and Uranium ($\text{U}$) exhibit variable and higher oxidation states ($+4,+5,+6$).