I have three cats with URIs that won't go away.?

Respiratory viruses can take a long time to pass, especially if they've got calicivirus. 

There are zero medications the vet can give you that will make this pass faster.  The only thing they can do is give you antibiotics IF they have secondary bacterial infections and any other meds to treat symptoms.

As far as hair loss goes, you'll have to talk to the vet about that.   Did the vet look at it under a black light to check for fungal infection (ringworm) or do a Fungassay culture?  Are the cats grooming each other to the point of harm?

As far as diarrhea goes, have you taken fresh stool samples to the vet to check for parasites?   If yes, are they actually sending the stool to the lab?

Tossing meds at a problem without doing proper diagnostic steps is generally the approach taken when either you or the vet is trying to keep the cost down.  Sometimes  it works, sometimes it doesn't.   There are tests to run for a reason.

maybe Corona Virus. I find a suspicious increase in URI symptoms of unknown origin being described in Cats on this site.

Our cat has just had a slightly 'bloody' pee and I believe he has a Urinary infection.  I will avoid giving him any dried biscuits and offer water and Natural Organic Yogurt that has 3 strains of bacteria, which may help him.  I read that fresh Parsley leaves, which are directic is good for cats, as well as powdered 100& Cranberries or fresh are a good treatment and Lingonberries.

Our cat has also been over grooming, since we took him to the vets last summer, about treatment for an infected gash on his head.  It was obviously very stressful for him and he hasn't got over it, yet.

If he doesn't improve by Saturday, we'll take him to the dreaded Vets!