If the parabola is sideways ("parallel to the x-axis"), then the equation is of the form "x = ay<sup>2</sup> + by + c". So plug in the three given points to get three equations in a, b, and c.
. . . . .9a + 3b + c = 3
. . . . .25a + 5b + c = 6
. . . . .9a - 3b + c = 6
Then solve the system for the values of a, b, and c. This will give you your equation.
Hope that helps a bit.
Eliz.