First after you build a Champion Cage it is highly reccomended to build a Champion Chamber afterwitch you can select one Champion and then freeze him in the Champion Chamber,keep doing this until you have Drull , Gorgo and Fomor.
Unlocking the right monsters
Having all three champions
Now you have to thaw one Champion and raise him to Level 6 Bonus 3 then freeze him and thaw a different one,keep doing this until all your Champions are Level 6 Bonus 3.
The right champion out
Thats about it.