That's too much of a coincidence for the two not to be related. It sounds like Pricilla was first, and after waiting a bit the person who did it got bold and returned to the scene of the crime to do it again with Denise.
The time gap between them and Sandee sounds like the person who did it was out of the area or in jail until 1984. Maybe the area where the first two went missing became more secure after the first two went missing, maybe the person forgot where they committed the crimes, who knows.
If the same person is responsible for all three it would make sense. If the person who did it was the guy who died in South Carolina was he in jail between 1975 and 1984? When were the two girls in South Carolina killed? Was it during that time period or after?
Can you solve the mystery of El Dorado, AR. Jane Doe?
AKA Cheryl Ann Wick
AKA Kelly Lee Karr
AKA Sharon Wiley