A quadrilateral has three acute angles each measures 80°.

Let $x$ be the fourth angle.

Since, the sum of all angles of a quadrilateral is $360^{\circ}$, we have:

$80^{\circ}+80^{\circ}+80^{\circ}+x=360^{\circ}$

$\Rightarrow 240^{\circ}+x=360^{\circ}$

$\Rightarrow x=120^{\circ}$

$\therefore$ The fourth angle is $120^{\circ}$.