Watery eyes

I found a cat about 2 weeks ago and brought it inside. I have 2 cats and kept the cat isolated for a week or so. I took it to the vet and the vet said it was healthy, so now it is around my other cats. Now my 2 cats have watery eyes. It's not constant, just every once in a while one eye starts tearing. They've never had it before this new cat was around. Should I take my 2 cats to the vet, or should I just wait it out?

Is it clear discharge or is it cloudy or gunky? Are they acting okay otherwise? eating, drinking, eliminating, playing?

They're acting fine. It's not gunky, just clear. It really looks like tears. And it's just one eye in each cat---from what I saw.

Also, the kitten's third eyelid (is that what it's called?) is showing. Could this be a sign of something?

You should play it safe and take them in to be checked. It is woefully hard to diagnose kitties over the Internet and without seeing them it is sometimes foolhardy to attempt it. The third eye showing is not normal and could indicate a virus is on board. Your vet would be the best one to guide you here. Good luck!