  A 3-graph $\mathcal{F}$ is \emph{$U(s, 2s+1)$} if for any $s$ edges
$e_1,...,e_s\in E(\mathcal{F})$, $|e_1\cup...\cup e_s|\leq 2s+1$. Frankl and
Kupavskii (2020) proposed the following conjecture: For any $3$-graph
$\mathcal{F}$ with $n$ vertices, if $\mathcal{F}$ is $U(s, 2s+1)$, then
$$e(\mathcal{F})\leq \max\left\{{n-1\choose 2}, (n-s-1){s+1\choose
2}+{s+1\choose 3}, {2s+1\choose 3}\right\}.$$ In this paper, we confirm Frankl
and Kupavskii's conjecture.
