Does the HP 3397 motherboard support RAM mixing?

As Blackbird has said, There isn't a motherboard or RAM module that guaranteed to work with any other if bought seperately. RAM is sold in kits for that reason.

Though while not supported, mixed RAM can work. You want identical models of RAM, and if you can't get identical, make sure they are the same in speed and timings. If they aren't the same, you'll have to manually set them in the BIOS.

Typically, unless you get a stick for free, just buy the kit