#### Answer

domain: $(-\infty, +\infty)$
range: $\left\{3\right\}$
Refer to the graph below.

#### Work Step by Step

RECALL:
A function whose equation is of the form $f(x) = a$:
(1) is a constant function, the graph of which is a horizontal line;
(2) all points on the line have a y-coordinate of $a$; and
(3) has the set of real numbers as its domain and $a$ as its range.
Thus, the graph of the function $f(x)= 3$ is a horizontal line. All the points on the line have a y-coordinate of $3$. Plot the points $(0, 3)$ and $(1, 3)$. (Refer to the graph in the answer part above.)
The domain is $(-\infty, +\infty)$ while the range is $\left\{3\right\}$.