I agree with Apognophos. It's a parable not a prophecy. FDS is like a straw boss. He only has power because the owner gave it to him. The Matthew account seems to suggest that FDS was given a little bit of responsibility and then when master came back and saw that things were in order, he was given more responsibility. Mark's account suggests that more than one slave was appointed over various things, like having a cook, gardener, maid, etc. The doorkeeper or butler was told to keep watch. Maybe he was in charge of the rest? Luke seems to support Mark with there being multiple slaves; the butler, maid, cook, and gardener all help keep watch, perhaps on a rotating schedule that the butler set up.
Great idea putting them all together! I agree with Londo111 as well. Parables are supposed to make one think about some issue, like the Good Samaritan.