  For a field $\mathbb{F}$, the notion of $\mathbb{F}$-tightness of simplicial
complexes was introduced by K\"uhnel. K\"uhnel and Lutz conjectured that any
$\mathbb{F}$-tight triangulation of a closed manifold is the most economic of
all possible triangulations of the manifold. The boundary of a triangle is the
only $\mathbb{F}$-tight triangulation of a closed 1-manifold. A triangulation
of a closed 2-manifold is $\mathbb{F}$-tight if and only if it is
$\mathbb{F}$-orientable and neighbourly. In this paper we prove that a
triangulation of a closed 3-manifold is $\mathbb{F}$-tight if and only if it is
$\mathbb{F}$-orientable, neighbourly and stacked. In consequence, the
K\"uhnel-Lutz conjecture is valid in dimension $\leq 3$.
