Harmonic, Melodic, and Natural minor?

I'm very confused with this. I have to write all three in the key of A. So I thought harmonic you lower the third and raise the 7th, and melodic you lower the third and raise the 6th and 7th. In an A scale, the 6th is an F sharp and I don't know how you would raise that.. would that be double sharped or am I doing something wrong?