1. The problem asks for the truth value of the proposition $Q(1, 2)$ where $Q(x, y)$ is defined as "$x = y + 3$."\n\n2. The formula for $Q(x, y)$ is $x = y + 3$. To find the truth value of $Q(1, 2)$, substitute $x = 1$ and $y = 2$ into the formula.\n\n3. Substituting, we get $1 = 2 + 3$.\n\n4. Simplify the right side: $2 + 3 = 5$. So the equation becomes $1 = 5$.\n\n5. Since $1$ is not equal to $5$, the statement $Q(1, 2)$ is false.\n\nFinal answer: The truth value of $Q(1, 2)$ is \textbf{false}.