the question is quite incomplete.
I am assuming that a thief is allowed to use the boat all by himself(i.e. without any policeman).

First,  X and his prisoner A go across.
X returns.
Y and his prisoner B go across.
A returns.
X and A go across.
B returns.
Z and C go across.
Y returns.
Y and B go across.
Thus they are all across the river.

Though it is highly likely that the prisoner would escape when left alone across the river or in the boat!!

first X & A go – return X
then B & C go – return C
then X & Y go – return Y & B
then Y & Z go – return A
then A & B go – return B
then B & C go – FINISH

question needs more clarity. A and X goes, then X returns.(if he doesn’t escape),
B and C goes, then C returns,
X and Y goes, then Y and B returns,
Y and Z goes, then A returns,
A and B goes, then B returns,
Band C goes. thus they returns.

This question seems incomplete or incorrect.

Anyhow my answer is:

first X & A go –  X returns
then Y & B go – Y returns
then Z & C go – Z returns
X & Y go – X returns
X & Z go.

Please clarify.