The following is the opposite:

1. Suppose A steals, A lies, B lies and C tells the truth.

Suppose B stole, A lied, B told the truth, and C told the truth.

Suppose C stole it, A is true, B is true and C is false.

Since there is only 1. In line with the requirements of the topic (only one of the three people told the truth), it can be judged that A stole the famous painting.

I hope the above answers can help you.