A set~$S$ of integers is a emph{$B_3$-set} if all the sums of the form~$a_1+a_2+a_3$, with $a_1$, $a_2$ and~$a_3in S$ and~$a_1leq a_2leq a_3$, are distinct. We obtain asymptotic bounds for the number of $B_3$-sets of a given cardinality contained in the interval $[n]={1,dots,n}$. We use these results to estimate the maximum size of a $B_3$-set contained in a typical (random) subset of~$[n]$ of a given cardinality. These results confirm conjectures recently put forward by the authors [emph{On the number of $B_h$-sets}, submitted].