I wouldn't blame Iron Maiden if they didn't want to open up for someone else because Maiden are so huge. Just goes with the territory. Now if this big 3 event did happen it should go like this...
Black Sabbath would be on first because they created metal. (not sure who would be the vocalist though) I guess if it has to be Ozzy then it has to be. lol
Judas Priest second because they had their big influence on metal also after Sabbath.
Iron Maiden third (headliner) because they are the biggest band of the three and also had a huge influence on metal.
Now this is just my opinion and no disrespect in the order. And if you want to throw Saxon and Motorhead (alhough their not metal according to some) in there as some kind of mini pre show thing to warm up the crowd then ok. lol
