The number of imaginary quadratic fields of class number 3 is finite ??

Question

I read here

The primes $p$ of the form $p = -(4a^3 + 27b^2)$

that

" It is known that the number of imaginary quadratic fields of class number 3 is finite. "

But the links did not show it.

And I know many class number questions are open for quadratic fields.

So is that claim correct ?

Reference or proof ?