I just ordered a book called "Three Ravens and Other Ballads", which is written specifically with accompaniment by lap harp. Sorry I don't know the publisher, I ordered it from Harp and Roses in B.C.
I quite agree with the earlier "less is more" comment. Especially if you are singing older songs, leaving out the third is modally accurate and also gives you a nice, clean sound. I do use full arpeggios for variety, though, at whatever line of the melody I want to have more dramatic impact.
Cindy Ellen Morgan