Acidity of $\text{CHX}_3$ compounds depends on the electron-withdrawing ability of the substituents X, which stabilize the conjugate base carbanion $\text{CX}_3^-$.

• CN is an extremely strong electron-withdrawing group (by both induction and resonance).
• $\text{CH(CN)}_3$ has three CN groups, creating maximum negative charge delocalization on the carbanion — giving the most stable conjugate base.
• Halogens: electronegativity order is F > Cl > Br > I, so CHCl$_3$ > CHBr$_3$ > CHI$_3$ in acidity.

Overall: CH(CN)$_3$ is the strongest acid due to the powerful electron-withdrawing CN groups.