I'd argue that Savage was a Sith. Maul declared himself a Sith, then took Savage as his apprentice. Debate the justification of whether the title is deserved or not all you like - but if you believe Maul to be a Sith, then Savage - legitly - is also a Sith.
Ventress is unique.
She was being used by Dooku with the lure of becoming a Sith apprentice, but never truly was one (even though Dooku claimed she was entirely). She's as much a Sith as Grievous (who was kind of in the same boat). You can argue Maul was more or less in that same boat, too.