Pardon my stupid question...
But, is it normal for a basset hound to have a visible third eyelid?
He was born with this condition. So far, it hasn't bother him at all. But, all his brothers and sisters does not have it.
I was just wondering if it can create a problem in the future
I'm pretty sure all dogs have this. Rosco and Layla both have one. They're more visible when they are sleepy. I believe it is called a nictating membrane and it usually can't be seen in other breeds, but in some dogs they are visible. It's supposed to keep the eye lubricated. Check the link below for more details. Unless it's really red or irritated I wouldn't worry.