Dropped D is easy to get into ad out of on the fly, and you don't have to relearn what to do on the other 5 strings, (as in DADGAD)

In a similar approach, when playing in G I like to drop the A string to a G--gives a nice solid drone, excellent for Scottish music. (Try flatpicking "Flowers of Edinburgh" this way, sounds great.) I don't know why this isn't more popular, its easy.

For one step more interesting, drop the low E all the way down to low C. Then you have a I-IV-V bass which can suport fingerpicking anywhere, in yer three chord config.

Bill