As far as I understand it, he had no intentions cold blooded killing Jon nor Dany. He had no intentions either getting any of the specific dragons. What he wanted was a dragon to fall under the water, so it could be taken up by the wights and be resurrected as a wight. If it would have fallen on the ground only, as Drogon would've, the other dragons could have burned it, and it would be of no use to the NK.
But sure, killing Drogon would also have killed them easier as well and the NK could of just keep throwing his spears until he killed the other two dragons. But I also think plot armour went into this. Dany and Jon has purpose in season 8, they would never kill them earlier obviously.
Nonetheless, this was a trap setup by the NK to go south. Chains and spears ready. But killing the most dangerous people along the way, Jon and Dany, would of obviously made things easier.
This might be tinfoil btw, but it's interesting how all the visions in the flames have led everything to this point. It almost feels like the red god is the night king, sending out visions to lead characters to this point, making him favoured to reach his target