Let's be honest, the audience doesn't care about the other two dragons. Drogon actually feels like he has a personality and is the one we see on screen the most. If Drogon died many viewers may have an emotional reaction, at least one more powerful than the lacklustre deaths of Viserion and Rhaegal.
So here's the fix: have Rhaegal and Drogon make it King's Landing just fine. Then on the day of the battle, have the Unsullied and Northern armies storm the walls and take out as many scorpions as possible. Once it is sufficiently safe to send the Dragon in, have Dany on top of Drogon bust down the gate while Rhaegal helps the Dothraki fight the Golden company (specifics aren't that important here). The battle turns in favour of Dany.
When the sky appears to be safe, suddenly a scorpion bolt will whizz past Dany. She looks down and sees that it came from a crowded residental area. More will fly into the air, and Drogon will do a good job dodging but ultimately it's not enough, and once one bolt hits he is vulnerable to more. The Dragon will manage to make it back over the walls to safety before collapsing and dying. Filled with rage, Dany will mount Rhaegal and go on a murderous rampage, ignoring the use of human shields, callously burning the hidden scorpions and innocent bystanders alike.
submitted by /u/StillFlyingHalfAShip