  Given a sixth power free integer $a$, let $E_a$ be the elliptic curve defined
by $y^2=x^3+a$. We prove explicit results for the lower density of sixth power
free integers $a$ for which the $3$-isogeny induced Selmer group of $E_a$ over
$\mathbb{Q}(\mu_3)$ has dimension $\leq 1$. The results are proven by refining
the strategy of Davenport--Heilbronn, by relating the statistics for integral
binary cubic forms to the statistics for $3$-isogeny induced Selmer groups.
