  Let $E\subset \mathbb{R}$ be a closed set of Hausdorff dimension $\alpha\in
(0, 1)$. Let $P: \mathbb{R}\to \mathbb{R}$ be a polynomial without a constant
term whose degree is bigger than one. We prove that if $E$ supports a
probability measure satisfying certain dimension condition and Fourier decay
condition, then $E$ contains three points $x, x+t, x+P(t)$ for some $t>0$. Our
result extends the one of Laba and the third author to the polynomial setting,
under the same assumption. It also gives an affirmative answer to a question in
Henriot, Laba and the third author.
