In addition to what Jim has said, the reason you want to span at least 3 bays is because you get the best, MINIMAL support when a piece of wood is supported at the 2 ends AS WELL AS the middle.
Also, I wouldn't attach any NEW wood with any old, rotted wood even if the disease/termites/whatever is dormant. Should there be any type of 'reoccurance', you don't want the 'old' wood to transfer its problems to the NEW wood.
There are lots of posts on this Forum that discuss things like downspouts, gutters, and landscaping to rectify possible drainage issues. (You'd be surprised at how many homes have problems here. You don't usually see them unless you're outside looking at these items while it's raining. Of course, put on a pair of boots and carry an umbrella to do so, whilst keeping your ears and eyes open for lightening.) Use your browser's FIND or SEARCH feature and look for keywords like 'french' or 'drain' or 'gutter'. In those Posts, as an aside, I (among others, et al.), talk about drainage.