Technically Crossfire as a name no longer exists. AMD has replaced it with Multi-GPU (or mGPU). And they only support it with two cards.
AMD used to have a compatibility chart for which GPU's can be mixed together. But the link is broken for it now. But I am pretty sure those two cards can be used together as they are in the same family.
However, the number of games that can utilize multiple GPU's is getting smaller ans smaller. So before you go and try it, see if any of the games you play are supported.
EDIT: I misread the secondary card as 570. With it being a 370, they are not compatible.