You can't tell what you will get, but there is a way to get what you want...sort of. The ore the spawns is chosen when you break the altar, not at world gen or killing WoF. Save the game right before breaking the first altar. If you get what you want, then save and reload. If not, then close out the game (a.k.a. hit the "X" at the top, do NOT use "exit and "save"). Rinse and repeat for the next two ores.
This worked last time I tried which was in 1.2.3. I don't think it's ever been changed.