The other three Discovery astronauts
It is never shown or stated in the film 2001, but in the
novel version, Dave Bowman ejected the bodies of the
three astronauts who were killed by HAL 9000 into space,
specifically into Saturn as I recall. I also recall that Bowman
sent Frank Poole's body towards the planet as well, and
felt a small sense of satisfaction that his friend would
make it to Saturn first.
Clarke himself said something to the effect that all four
2001 novels operate in their own parallel universes, which
is why they have some rather glaring inconsistencies
(more like loose ends that remain unresolved in a few
cases).
So if this is the case, then maybe those three astronauts
are still floating out there in the void, waiting for their own
novelizations.