I recently ran into that error message with a companion voucher. I didn't realized I had selected partner flights rather than strictly Westjet flights. Once I removed the partner flights my voucher worked. I also asked on Facebook if I could book a stopover and they told me to call in. As a pp said, I'd be surprised if your voucher would get you three flight legs, though. In my experience a stopover is 24-48 hrs, though admittedly I haven't used that option in years. I get pretty quick results by asking my questions via FB messenger to Westjet's page.