"We've agreed to two more [seasons,] but I could get into trouble for saying that," Cumberbatch told Radio Times.
"All I know at the moment is we're doing these three [TV movie-length episodes for Season 3] and another three. I just don't know what there is beyond that. ... It just depends on Martin and [my] availability, how long we can keep it going. It depends on Steven's ability. I'd love to keep it going."
"Sherlock" showrunner Steven Moffat is also the lead writer-producer on "Doctor Who."
Production on Season 3 of "Sherlock" is to begin Monday. It is expected to air this year.
Cumberbatch and Freeman are both starring in popular film franchises. Cumberbatch will soon be seen in "Star Trek: Into Darkness" and Freeman is the star of Peter Jackson's "The Hobbit" trilogy.