I completely understand your skepticism, and that "3" isn't that old, but chickens don't have near the lifespan as even dogs and cats unfortunately. I am not trying to be a negative Nancy, just being realistic and keeping things in perspective. I recently went through this with a very sweet 7...
To be completely honest, a lot of times, especially with older hens, crop issues are a side affect caused by reproductive issues. What is their feed brand exactly so we have all the necessary information. @azygous @Wyorp Rock @Eggcessive These folks are much more knowledgeable than me
I'm sorry to hear that your hen is doing poorly! Is there a smell coming from her beak? Like a pungent yeast smell? Give this a read, then reply with anything further you've discovered. I'd switch to coconut oil for the crop messages, chill it in the freezer then break off little pieces and feed...