How to create 3-elements dragon?

One way you can get a 3 element dragon is to breed War and Mud. You have ~20% of getting a Quake dragon which is War, Terra, and Water. You have to breed it in the Breeding Sanctuary lvl 2

yes lvl is dependent and it also depends on what device your playing on and on what lvl the breeding sanctuary is

I suggest you lvl up on of the parent dragons you're going to breed to lvl 15 and one could be at 10 or higher but one must be at lvl 15. that's how I got the apocalypse. and your sanctuary might breed differently if you play on facebook or on mobile and usually mobile gives the best results when bred directly

so mud/waterfall + war would give you a better chance of getting quake on mobile than on facebook which requires a lot of different elements and I usually never get the quake by breeding directly so mobile is better

It is possible to get a three-element dragon without using the breeding sanctuary(which is locked by your level by the way). However, you have to use a lvl 20 dragon parents to breed those dragons, such as the poo dragon, the angry dragon, and the crystal dragon.