The least perfect square number which is divisible by each of 21, 36 and 66 is

$21=7^1\times 3^1$

$36=2^2\times 3^2$

$66=2^1\times 3^1\times {11}^1$

L.C.M of  21,36,66 is $=7^1\times 2^2\times 3^2\times 11$

To make it perfect square multiply 77

$=7^2\times 2^2\times 3^2\times {11}^2=213444$