Dragonite
My Dragonite data at: https://pokemongo.gamepress.gg/q-a/another-dratini-question.
Today, I tested my Dragonite at gym defense, it worked pretty bad, though.
First, I used an CP 728 Onix with T/RS, it works pretty well against the Dragonite, still having about half of its HP left. Note: the Onix's IV is slightly better.
Then, I tested with a steel Pokémon, not Magneton, was Magnetmite! It is 747 CP and has a pretty decent IV. The Magnemite won the match with about 1/7 of its HP left.
To be sure that type effectiveness affects only a small part of the matches, I used a 677 CP Bulbasaur with a combo of grass moves. The Dragonite won the match, but only having less than half of its HP left. The Bulbasaur has a decent IV.
The prestigiers all have 4000 Stardust cost.
So the question is: this very disappoints me from powering up my Dragonite. Should I really power it up?
I know the biggest effect of these matches was the level between Pokémon, and DB/DP is not the best moveset defending.
Answers
Currently CP 920.
Ok, you have tested your CP920 Dragonite with a 728 Onix, a 727 Magnemite and a 677 Bulbasaur.
How often will your CP3000 Dragonite be confronted with a CP2300 Onix, a CP2300 Magnemite and a CP2200 Bulbasaur?
I am pretty sure everybody could kill a dragon with those PKM. I doubt, that you have chosen a very realistic scenario.
In a fair fight, a 900 dragonite is the same as a 900 Onix. Because of human error and AI randomness, different things can happen. CP tries to encompass the three base stats into one number that can compare it to another mon - although it definitely is not perfect. It doesn't take into account moveset or type advantage/disadvantage. I think the current CP formula is terrible as it highly values attack stat over def and stamina.
DB/DP isn't the best moveset because the AI doesn't take advantage of DB's fast cooldown. Dragonite also doesn't make a great prestiger because at a low CP, its attack greatly outweighs its Def and Stamina.