Friction and those thumb shifters are basically the same, except the thumb shifter has a ratcheting mechanism, so it "clicks" as you move it. Both are not "Indexed." When using it on S3X, my guess is that you just move the shifter until you "feel" the next gear, then stop. Pretty simple.
Here's a bargain you can't go wrong with. I have a set of these and I would think they would get the job done. At 99 cents plus shipping to Down Under, it's worth a try.
http://www.nashbar.com/bikes/Product..._200276_200468
