] I suspect that this, in addition to the reasons Mark guessed at and Dee also laid out, made it a financially logical thing to do in those times.

I hadn't seen Henry's interesting post, but I did find similar information at the Victorian Web site. I know it's out of period, but I'll put the link below anyway, as the three-volume formula obviously existed prior to that period.

The VW site explains how Mudie's Lending Library influenced the format of literature for over half a century. The article begins:

Between 1842 and 1894 Charles Edward Mudie's lending library influenced Victorian literature, particularly fiction, in two chief ways: first, by making sure that almost all novels appeared in three volumes, it had important effects on the structure, plot, style, and even imaginative worlds of the Victorian novel; and second, by acting as a censor who demanded fiction suited to the middle-class family, it controlled ther subject, scope, and morality of the novel for fifty years...

The rest of the article can be found HERE.