The number divisible by 11 is

A natural number is divisible by  ‘11’ if and only if , the difference of the sum of the numbers obtained on adding the alternate digits of the numbers separately is divisible by 11.

$333333$ is divisible by 11.

Since,  sum of the digits in odd places $=3+3+3=9$

             Sum of the digits in even places $=3+3+3=9$

difference $=9-9=0$ is divisible by 11.