To evaluate the simplified expression $3x$ with $x = 5$, substitute 5 for $x$:

$3x = 3 \times 5$

$3 \times 5 = 15$

Therefore, when $x = 5$, the value of the expression is:

$\boxed{15}$

Would you like more details or have any questions?

Here are 5 related questions for further practice:

1. What is the value of $4x + 2$ when $x = 5$?
2. If $x = -2$, what is the value of the expression $3x$?
3. Evaluate $2x^2 + 3x - 5$ when $x = 5$.
4. Substitute $x = 3$ into the expression $4x - 7$ and find its value.
5. If $5x - 10 = 15$, what is the value of $x$?

Tip: When substituting values into an expression, ensure all occurrences of the variable are replaced to avoid calculation errors.