With both Batista and Orton as heels, the WWE has concerns over how the fans would respond to the match. The idea of a third entrant will only serve as a way for the WWE to appease the fans and put their hero Daniel Bryan into the world title match.
As of this article, Daniel Bryan is currently looking at a potential match against Triple H at WM 30. A possible scenario could see Bryan defeat HHH at WM 30 and earn the chance to be in the world title match with Orton and Batista.
Other potential undercard matches are:
Big E vs. Dean Ambrose to unify the secondary titles
The Shield (Reigns and Rollins) vs. The Wyatts (Harper and Rowan)
Sheamus vs. Christian
Goldust vs. Cody Rhodes
A match consisting of NXT stars
The only matches that are locked in for WM 30 right now are:
Cena vs. Bray Wyatt
The Undertaker vs. Brock Lesnar
Orton vs. Batista for the WWE World Heavyweight title