Mass dragon is too expensive to use for farming, so I only use on Clan Wars or a rare revenge situation. To 3-star the base you need to be able to take out the air defenses easily, so if their air defenses are super D-ed up (behind too many layers of walls or otherwise defended) then the strat won't work. So I look for a base that has air defenses that can be relatively easily taken out or overwhelmed by the dragons.
First and ALWAYS successfully pull the clan castle and kill it. A pack of wizards or even archers will crush mass dragon attack. So learn how to deal with the CC first. Once that's done then send in some giants or hogs to take out the air defense (presuming that they are relatively undefended and can be quickly killed -- can't waste tons of time on it or the dragons won't have time to 3-star him). I've resorted to triple lightning on an air defense that was super D-ed up but that's a rare case and wont usually work because that only handles one air defense and uses too many spells. I like to use rage spells and one heal spell with mass dragon.
OR... if the air defenses are low level and near the edge of the base, you can get away with using the dragons themselves to kill the air defense, but its risky because dragons have no priority and can easily go chasing a row of pumps/mines while the air defenses are just raping your birds (I've seen it happen!).
Lastly, don't drop all your dragons on the same spot (i.e. don't hold down your finger). Drop two on one spot, then two more next to them, then two more next to them. So its a row of dragons but two at a time rather than all at one spot. The reason is that they'll all attack in unison so you'll have 6 dragons bearing down on one canon (for example), but it only needs two dragons to kill it, so the other 4 are wasting their attacks on a thing that's already dead. (BTW this goes for wizards too; a pile of wizards on one spot will act in unison and waste their attacks, plus they are more vulnerable to AEO attacks from mortars or Wizard towers etc.)
Hope that helps.

