Excluding their marriage, were Prince Charles III and Lady Diana Spencer related?
Correct answer: Yes

Tom
Princes are not given numbers. "Prince Charles" is correct, "Prince Charles III" is wrong.
Kings have numbers
"King Charles" or "King Charles III" , to distinguish from other Charles are both ok.

Yana
Is anyone their own grandpaw ????!!

I am not a number.
King Charles is his own fourth cousin because Queen Elizabeth and Prince Philip were third cousins. He is also his own third cousin once removed.

Sassy
Could that be why they never got along?