check the picture below.
now, keep in mind that the focus point is at 3,0 and the directrix is to the left-hand-side of it, therefore, is a horizontal parabola, and it opens to the right-hand-side, like in the picture.
keep in mind that the vertex is half-way between the focus point and directrix, at a distance "p" from either one, notice the "p" distance is just 3 units, since the parabola is opening to the right, "p" is positive.
[tex]\bf \textit{parabola vertex form with focus point distance}\\\\
\begin{array}{llll}
\boxed{(y-{{ k}})^2=4{{ p}}(x-{{ h}})}
\\\\
(x-{{ h}})^2=4{{ p}}(y-{{ k}})
\end{array}
\qquad
\begin{array}{llll}
vertex\ ({{ h}},{{ k}})\\\\
{{ p}}=\textit{distance from vertex to }\\
\qquad \textit{ focus or directrix}
\end{array}\\\\
-------------------------------\\\\
\begin{cases}
h=0\\
k=0\\
p=3
\end{cases}\implies (y-0)^2=4(3)(x-0)\implies y^2=12x\implies \cfrac{1}{12}y^2=x[/tex]
