Two sides of a triangle are 3 cm and 8cm. If the triangle is isosceles, then the possible value of third side is

In $\mathit{∆}PQR$ sum of $PQ+PR=3+3=6<8$ it cannot form a triangle

In $\mathit{∆}ABC$ sum of any two sides greater than the third side

$3+8=11>8$

$8+8=16>3$

Difference of any two sides is less than third side

$8-8=0<3$

$8-3=5<8$

$\therefore$The possible value of $3^{\mathrm{rd}}$ side is 8 cm.