There is no definitive answer in the movie*.
We do not see what happens to Willis or Walker but it's certainly implied that they are executed.
When news of their capture is relayed back to NASA they are always referred to in the past tense and when the helicopter pilots attempt to stop Brubaker from escaping they have no hesitation in using deadly force to do so.
The fact that we do not see all three astronauts at the end lends further weight to the theory that Brubaker was the only survivor.
*There were TWO novelisations of the movie (a UK version and a US version) which may add more detail but I have not been able to locate an e-book copy of either to verify.