$第一個(gè)數(shù)是\dfrac{1}{3}，用\dfrac{1}{1}\times \dfrac{{\displaystyle 1}}{{\displaystyle 3}}表示；$
$第二個(gè)數(shù)是\dfrac{1}{6}，用\dfrac{1}{2}\times \dfrac{1}{3}表示；$
$第三個(gè)數(shù)是\dfrac{1}{9}，用\dfrac{1}{3}\times \dfrac{1}{3}表示；$
$第四個(gè)數(shù)是\dfrac{1}{12}，用\dfrac{1}{4}\times \dfrac{1}{3}表示；$
$由此可知，第幾個(gè)數(shù)就用幾分之一乘三分之一，$ $第n個(gè)數(shù)，就是\left(\dfrac{1}{n}\times \dfrac{1}{3}\right)$
$第五個(gè)數(shù)是：\dfrac{1}{5}\times \dfrac{1}{3}=\dfrac{1}{15}$
$第六個(gè)數(shù)是：\dfrac{1}{6}\times \dfrac{1}{3}=\dfrac{1}{18}$
$第七個(gè)數(shù)是：\dfrac{1}{7}\times \dfrac{1}{3}=\dfrac{1}{21}$
$故答案為：\dfrac{1}{15}；\dfrac{1}{18}；\dfrac{1}{21}$
