The GCD of given numbers is 222.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}666 =& 2\cdot3\cdot3\cdot37\\[8pt]888 =& 2\cdot2\cdot2\cdot3\cdot37\\[8pt]1110 =& 2\cdot3\cdot5\cdot37\\[8pt]\end{aligned}$$(view steps on how to factor 666, 888 and 1110. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}666 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot\color{Fuchsia}{\boxed{37}}\\[8pt]888 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot\color{red}{\boxed{3}}\cdot\color{Fuchsia}{\boxed{37}}\\[8pt]1110 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot5\cdot\color{Fuchsia}{\boxed{37}}\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 2\cdot3\cdot37 = 222 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.