To shift the function \( f(x) = 2x^2 - 1 \) to the left by 3 units, you replace \( x \) in the function with \( x + 3 \).
So, the shifted function \( g(x) \) can be written as:
\[ g(x) = f(x + 3) = 2(x + 3)^2 - 1 \]
Now, looking through the provided options:
- \( 2(x-3)^2 - 1 \): This represents a shift to the right, not the left.
- \( 2(x+3)^2 - 1 \): This is the correct representation of the leftward shift.
- \( 2x^2 + 2 \): This is not correct, as it does not represent a shift of the original function.
- \( 2x^2 - 4 \): This is also not correct for a leftward shift.
Thus, the correct expression for \( g(x) \) is:
\[ \boxed{2(x+3)^2 - 1} \]