Replace rotted wooden sill of the door?
It's called a threshold and 36" is a standard size but many places might carry a larger size that has to be cut to fit due to door framing. Your best bet would be to remove it and take it with you to a home store and get a close match, they can vary a small bit in style but the height needs to be the same as it will fit up against your door. I have always caulk around any threshold I've installed just as an added protection against moisture. Good luck