War of The Ring (SPI, 1977) In a three player game, can two players actually win at the same time?
Subsection W.2 of the rules gives the procedure for Saruman to gain control of Nazgul.  Halfway through the first paragraph it says:

... If Saruman loses the Ring, the Nazgul that he controls remain with him. ...

Saruman's victory condition requires that he control all surviving Nazgul.  If he does that and subsequently loses the ring, then he would retain control of the Nazgul he had captured.  Therefore, if he also held Isengard, Helm's Deep, and Edoras he would fulfil his victory condition, even if the Fellowship player managed to destroy the ring.  Note that in this unlikely scenario, Saruman himself would be dead, since section M.1 provides (emphasis original):

A Ringbearer may never voluntarily surrender the Ring.  The only way a different Character can become the Ringbearer is if the first Ringbearer is killed.

This restriction is reaffirmed for the three-player game in subsection X.2.
Note, however, that if all of the Nazgul were destroyed, then Saruman could fulfil this part of the victory condition without ever having possessed the Ring, and could therefore be alive for a shared victory.  That is, it appears to be possible for Saruman to ally with the Fellowship player and play explicitly for a joint victory.
The only remaining question is whether it's possible to have these events happen simultaneously without the game ending first.  Section U gives the victory conditions for the Campaign Game, but it is vague about when those military victory conditions are checked (for ring victory conditions, Section U references the Character Game victory conditions in section O, which state that the Fellowship ring victory condition is checked at the beginning of the Ring Phase). Section O does state in its preamble (outside of any subsection) that in the Character Game, the game ends immediately when a victory condition is met, but since section U references subsections O.1 and O.2, but not section O in its entirety, it's unclear whether that provision applies in the Campaign Game (the three-player option is only available in the Campaign Game).  Section Z, which gives the modified victory conditions for the three-player game, does not clarify this matter.
In summary, a joint Fellowship-Saruman victory appears to be possible, provided that satisfying military victory conditions does not end the game immediately.  The fact that the rule writers explicitly called out the possibility of a joint victory suggests, albeit indirectly, that this is the intended interpretation.