(1) the terrudans will kill each other
Possible, but certainly won't Volition refuse to anger the Zod fans?
(2) shivans will kill terrudans
In which case, the player's race is doomed. Not possible, though not impossible.
, (3) terrudans evolve beyond shivans and tame shivans

The only way I imagine Terrudanity doing this might be via an Ancient-based "fling a light into the future" weapon or something of the sort, something mystical that can kiss the ass of even the most menacing species in the galaxy (recall Halo's extermination of the Flood using the Halo Array). That said, Sushistorian will most likely be gratified

(4) terrudans kill shivantown
Like I said, this will probably come out with some sort of Ancient-based superweapon or some kind of mystical Divine Intervention by whatever elderly race.
(5) all the three species go extinct by themselves or in the midst of a war between them

like the first one.
(6) a fourth species comes and wipes all the other three out
Hmm...Nightmares, anyone? This can be possible, but again, the player's own race be wiped out??
(7) some environmental disaster ends the conflict
Certainly no if by mere chance, and by surprise. It will be

for the Freespace community.
(8) the universe ends in a nanosecond without notice.
Let us all create a mega-supernova SEXP in FRED!!!