To expand the expression \((x+3)^3\), you can use the binomial theorem or simply expand it step-by-step. Here’s the step-by-step method:

\[
(x+3)^3 = (x+3)(x+3)(x+3)
\]

First, expand \((x+3)(x+3)\):

\[
(x+3)(x+3) = x^2 + 3x + 3x + 9 = x^2 + 6x + 9
\]

Next, multiply the result by \((x+3)\):

\[
(x^2 + 6x + 9)(x+3)
\]

Distribute each term in the first polynomial by each term in the second polynomial:

\[
= x^2(x) + x^2(3) + 6x(x) + 6x(3) + 9(x) + 9(3)
\]

\[
= x^3 + 3x^2 + 6x^2 + 18x + 9x + 27
\]

Combine like terms:

\[
= x^3 + 9x^2 + 27x + 27
\]

So, the expanded form of \((x+3)^3\) is:

\[
x^3 + 9x^2 + 27x + 27
\]