If The Munsters only lasted 2 seasons how come they had 3 diffrent startings?

They had 3 opening credits, because after 13 episodes Beverley Owen left the cast. They had to re-shoot the Season 1 credits to now include Pat Priest. That's why there are two different credits in Season 1.

When Season 2 came along, they changed the credit opening, but still with the same cast.