Let the congruent sides of an isosceles $△$ be $x$.
Let the base of the isosceles $△$ be $y$
According to the 1st condition,
$x+x+y=30$
$2x+y=30$
According to the 2nd condition
$x=y+3$
$x-y=3$
Solving equation (1) and (2)
$2x+y=30$
$x-y=3$
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$3x=33$
$x=\frac{33}{3}$
$x=11$
By substituting $x=11$ in equation (1)
$2x+y=30$
$2(11)+y=30$
$22+y=30$
$y=30-22$
$=8$
$\therefore$ The length of the sides of a $△$ are $11cm,11cm$ and $8cm$.