I will try with the following statement.

"The treasure is in the left chest or it is in the middle chest and this sentence is false."

If the treasure is
- in the left chest: the sentence is true regardless of the second part.
- in the right chest: both parts of the sentence are false.
- in the middle chest: it claims that the sentence is false. Paradox.

I will try with the following statement.

"The treasure is in the left chest or [it is in the middle chest and this sentence is false]."

If the treasure is
- in the left chest: the sentence is true regardless of the second part (after "or").
- in the right chest: both parts of the sentence are false.
- in the middle chest: it claims that the sentence is false. Paradox.