To get a third order polynomial in x (x^3), you can do
lm(y ~ x + I(x^2) + I(x^3))
or
lm(y ~ poly(x, 3, raw=TRUE))
You could fit a 10th order polynomial and get a near-perfect fit, but should you?
EDIT: poly(x, 3) is probably a better choice (see @hadley below).