Daenerys suddenly shows up over New York City (our world) one morning with three dragons. She goes full "King's Landing" and attacks, striving for as much carnage as possible.
Assume neither Daenerys nor the dragons get tired, nor run out of dragonfire. They keep going until they're killed or NYC is entirely smoked.
Can she be stopped?
How long before she's stopped?
How is she stopped?
How much damage does she do before she's stopped?
Fight #1: NYC not allowed outside help.
Fight #2: NYC can call for outside help.