#### Answer

The given relation does not define $y$ as a value of $x$.
domain: $(-\infty, +\infty)$
range : $(-\infty, +\infty)$

#### Work Step by Step

RECALL:
A function is a relation where every value of $x$ is paired with only one $y$ value.
Substituting $0$ to $x$ gives:
$x+y \lt 3
\\0+y \lt 3
\\y \lt 3$.
This means when $x=0$, $y$ can be any real number less than $3$.
Thus, when $x=0$, the points $(0, 2), (0, 1), (0, 0)$, plus a lot of others satisfy the inequality.
An $x$ value is paired with more than one value of $y$, then the given relation does not define $y$ as a value of $x$.
$x$ and $y$ can take any value therefore:
domain: $(-\infty, +\infty)$
range : $(-\infty, +\infty)$