In actuality, the old saying "2 grooves means Confederate and 3 grooves means yankee" is merely a rule-of-thumb, because it applies ONLY to the very-most-common yankee minies and Confederate minies. The McKee-&-Mason book on civil war bullets shows nearly 50 varieties of Confederate 3-groove minies.
Rocky 77, the photo shows your bullet is very corroded, which makes it difficult to tell whether it is a US or CS one. Although we can still remnants of 3 body-groves, we'll have to rely on the shape of its base-cavity. Is the cavity the usual simple wide cone shape, or some other shape (such as a "cup-cavity)?
Please post a photo showing the base-cavity's shape. If it is a simple wide cone, there's no way to tell whether your very-corroded civil war bullet is a US one or a CS one.