You can use a flea comb on the 3rd cat.

You comb slowly through the cat's fur, & fleas can't slip through the close-set teeth.

Keep a bowl of soapy water close by, & dunk the fleas into that.

It's very easy, & the cat will love it.

You should really let us know what product you used. You mentioned you bought it from Amazon, but it is not a grocery brand.
Your cat might be reacting to the smell....my friends Chihuahua hates the smell, a couple of hours will go by and he is back to normal, but the minute I put it on and for hours after he acts really weird.

I've tried both FrontLine Plus and Advantage.

I think you may be right about the smell. He's kind of a weirdo about certain things. My other 2 love when I give them laxatone but he runs like a bat out of hell if he smells it.

Thanks for the responses!

Try Diatomaceous Earth, often called fossil flour. (food grade only!) It has always worked wonderfully for us for chickens, dogs and cats. It kills mites, fleas, ticks. When I see a problem, I dust them or put it in their ears. I don't normally have to use it at all in the winter even though they are outdoors as well as inside when it is too cold.

I also would worry about chemicals being absorbed through the skin. DE is perfectly safe and can be ingested without harm.

DE works to some extent as long as you don't have an infestation already. I haven't found it to work much at all as the only treatment in an infestation situation.

FYI, Frontline does not enter the bloodstream like all other flea/tick products. It stays in the skin, dermis, and sebaceous glands then coats the fur from there. All other products are absorbed into the bloodstream which is why I personally use Frontline Plus. For my clients, I recommend it until/unless their fleas have become resistant, but it really works much better when used every month as directed. When you use it only when you see fleas, you run the risk of creating insecticide-resistant fleas.