Thanks for your replies. Since there are different opinions on the subject, I decided to ask support directly. Promptly received a Ticket ID and answer:
I posed the question a little differently, mentioning that I have three computers, two with only one OS each and one with three OSs in three separate partitions. For licensing and pricing considerations, does this count as three devices, or five?
Answer: "For such cases, we consider it as three devices."
I'm happy with that.