Out of curiosity, I checked with Barnes & Noble, and found out that they're still available as Nook books (besides as print books, I'm sure). Since I do all my book reading using the Nook app on my tablets, I decided to read them again. The first time I read them, it was as paperbacks, before I even knew anything about ebooks.
It surprised me that, although Red Mars cost a typical $3.95 (for an ebook), Blue Mars was $12.95 (!), and Green Mars $5.95. Still, just about $25 for all three, which I still consider a bargain.