Sure, why not? Isn't that one of the very first combinations you do when learning international foxtrot -- Feather, Three, Feather, Three . . . straight down the room? My books aren't with me, but if they don't list the Feather Step as a follow to the Three Step I can make a couple guesses on the reason.
It could be alignment. I believe a Feather is typically taken somewhere around DC while a Three Step is typically taken somewhere around DW. Following up a Three Step DW with a Feather Step DW might not be a very good combination. It could also be the way the ISTD describes figures. They have a nasty tendency to include the first figure of the next figure as the last step of the currently described figure. They might have describe the Feather and Three in such a way that you couldn't follow the Three Step with a Feather Step by their descriptions because you'd be on the wrong foot or something. Hmm, those are actually the only two reasons I can think of. Maybe if I remember, I'll look at the book when I get a chance and see if I can make some more sense out of their descriptions. But I hope this is of some help for now.
