The GCD of given numbers is 3.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}3 =& 3\\[8pt]1200 =& 2\cdot2\cdot2\cdot2\cdot3\cdot5\cdot5\\[8pt]\end{aligned}$$(view steps on how to factor 3 and 1200. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}3 =& \color{blue}{\boxed{3}}\\[8pt]1200 =& 2\cdot2\cdot2\cdot2\cdot\color{blue}{\boxed{3}}\cdot5\cdot5\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 3 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.