there is a form of the tubular cast on that uses 3 needles.
Cast on 1/2 of the total, using a simple cast on . (use needle A the largest)
knit a few rows of stocking knit (needle B the intended size to work with)
Pick up stitches from cast on edge (use needle C the smallest)
hold needles (C in back B in front--with the KNIT side of the stocking knit facing you (and needle B facing you)
Knit 1 from needle B (on to a second B needle)
Purl (or knit) one from needle C on to second B needle)
repeat these two steps till all stitches have been knit (you should end with a B needle stitch (an odd number!)
since when you pick up stitches from a cast on edge, you get 1 fewer.
this is the ONLY 3 needle cast on I know. (and i know lots of cast on's)
Now days, i do a tubular cast on differently.
(there are to my knowledge 5 or 6 (depending on which way you count) ways to do a tubular (also known as an italian) cast on.
[I]you could count a magic (use 2 needle to do a magic cast on) and then knit the stitches together onto a third needle as yet another way to do a tubular/3 needle.
[/I]Or as Suzeeq suggest, it could just be a circular cast on that starts using just 3 of (4/5) dpn's