# Multivariate rational function field of rank 3 over Rational Field

I'm want to rewrite a small Magma code in Sage, and I want to define a multivariate rational function field of rank 3 over rational field. Simply, I have the following Magma code:

```
aInv<a2,a4,a6> := FunctionField(Rationals(), 3);
_<X> := PolynomialRing(aInv);
f := X^3 + a2*X^2 + a4*X + a6;
fprim := Derivative(f);
```

I started doing something like this:

```
aInv<a2,a4,a6> = FunctionField(QQ['a2,a4,a6'])
```

But, I get a TypeError, more specifically, I get:

TypeError: constant_field must be a

Any idea how to achieve what I want?

field.