The GCD of given numbers is 5.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}15 =& 3\cdot5\\[8pt]25 =& 5\cdot5\\[8pt]5 =& 5\\[8pt]\end{aligned}$$(view steps on how to factor 15, 25 and 5. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}15 =& 3\cdot\color{blue}{\boxed{5}}\\[8pt]25 =& \color{blue}{\boxed{5}}\cdot5\\[8pt]5 =& \color{blue}{\boxed{5}}\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 5 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.