Well, the picture shows one battery pack.
The title says, "...battery pack..."
The Item specifics says, "A brand-new...item...."
The description says, "For sale one: ...battery...."
I would think that if there was any confusion, it would be that you were selling only one of the three batteries in the pack, not three of the battery packs.
Maybe next time leave out the spaces on either side of the "x," put a "#" sign in front of the "NS," and use commas to contain the the part number so the ending of it can't be construed to be a quantity indicator, and add the word "pack." Maybe even add the word "brand."
FOREVER (brand), Rechargeable Ni-MH Battery Pack,
#NS-AAA600mAhx3, 3.6V 600mAh.
Different people see things differently. It's always better to avoid ambiguity to begin with, than to wonder if someone really misunderstood, or is trying to rip you off; and then waste your time dealing with it.