To get the Deep Red Dragon(Flame + Metal + Dark), you will need to unlock level 1 breeding sanctuary for 25 gems. Then, you can use the following combinations to get it:
- Petroleum + Medieval
- Medieval + Dark
- Pearl + Dark Fire
The main point is to make sure to have all 3 elements in the breeding combination. Just make sure to have all 3 elements(Flame,Metal,Dark) in it. For more info, check out the breeding guide.