  Let $f\in \mathbb{R}[x, y, z]$ be a quadratic polynomial that depends on each
variable and that does not have the form $g(h(x)+k(y)+l(z))$. Let $A, B, C$ be
compact sets in $\mathbb{R}$. Suppose that $\dim_H(A)+\dim_H(B)+\dim_H(C)>2$,
then we prove that the image set $f(A, B, C)$ is of positive Lebesgue measure.
Our proof is based on a result due to Eswarathasan, Iosevich, and Taylor
(Advances in Mathematics, 2011), and a combinatorial argument from the finite
field model.
