These aren't lyric sites, but they're often a good starting point when searching for songs:

AMG - All Music Guide lists 2 recordings called "3 Sisters" and 34 called "Three Sisters" -- then there are variations, like "Three Sisters Waltz," "Three Little Sisters," etc. If you follow the links, it will sometimes bring you to Barnes&Noble which sometimes has sound samples.

Or you can start at Yahoo! Shopping > Music, which has more samples and takes you to them by a more direct route. There, a search for "Three Sisters" finds 115 recordings, lumping together all the variations, like "Three Little Sisters."

Often, if you find sound sample of the right song, and transcribe as much as you can from it, and then use Google to search for those words, you will find the complete lyrics, if they exist anywhere on the searchable part of the Internet. (And don't forget to search Mudcat, too.)

Or if that fails, post whatever information you find (lyric quotes, author/composer, artist names, album titles) back here and it may jog someone's memory.