Which of mono-, di-, triphenylmethane is the most acidic?
You should consider the stability of carbanion that is left behind, after removing $\ce{H+}$.   One could make arguments for resonance stabilization of each of the carbanions. You could "push" the minus charge around the phenyl rings, and there is a general rule that the more "good" resonance structures that one can draw, the more stable the molecule.

It turns out that the tertiary carbanion in 2) is flanked by three phenyl groups (thus many good resonance structures compared to 1) and 3) ).