The GCD of given numbers is 1.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}2015 =& 5\cdot13\cdot31\\[8pt]3 =& 3\\[8pt]2018 =& 2\cdot1009\\[8pt]3 =& 3\\[8pt]\end{aligned}$$(view steps on how to factor 2015, 3, 2018 and 3. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}2015 =& 5\cdot13\cdot31\\[8pt]3 =& 3\\[8pt]2018 =& 2\cdot1009\\[8pt]3 =& 3\\[8pt]\end{aligned}$$Note that in this example numbers do not have any common factors.
Step 3 : Multiply the boxed numbers together:
Since there is no boxed numbers we conclude that $~\color{blue}{ \text{GCD = 1} } $.