We know that the probability of selection of $r$ objects out of $n$ distinct objects when $p$ objects are always selected is ${}^{n-p}{C}_{r-p}$.

Here, $n=8, r=3, p=1$

The number of ways of distributing eight identical rings to three different girls so that every girl gets at least one ring is ${}^{8-1}{C}_{3-1}$.

$\Rightarrow {}^7{C}_2=\frac{7!}{5!\times 2!}=21$ ways.