Sorry, but I really don't know what Warcraft is.
However, if it is a d20 gaming product compliant with the OGL, they have the legal obligation to tell one must have the PHB to use it. Now, their own rules are probably made compatible with 3.5, so they say "needs 3.5". But the truth is, that many d20 games (such as Traveller 20 or Grim Tales for examples), do not require the core rulebooks once you know the game. If you know the basics (roll stats for character, and the basic mechanics) you can buy the game and play with it, it doesn't require anything. In any case, since you have 3.0 you already have all you need to use this Warcraft.
Of course, since you have 3.0, you can also download the SRD 3.5 on WotC website, and thus you are sure to have everything needed to use Warcraft (just in case).