So if the child of a god and mortal is a demigod, and the children of demigods are called legacies (or demigods), then what would the child of a god and demigod be called? They would be three quarter god and only one quarter human so would they still be a demigod or something else?
They would still be a legacy, or possibly a demigod.
The wikia states
A legacy is any mortal who is a second or further generation descendant of a god. They may or may not possess similar abilities to their godly ancestors.
Demigods, or half-bloods, are a race of beings who are half-mortal, half-god. They possess mortal souls and are vulnerable to old age and death;
So the demigod in question is a direct result of a god/mortal pairing. No matter how far you go down the line, that person will always have mortal DNA. So, most likely a legacy since any kids would be 2nd generation from the demigods parents, or possibly a demigod.
It isn't spelled out, so I look at it as being a legacy, since it's not the first god/mortal pairing in the mix.