There was a discussion about a tach for the R3 a while back.
Apparently its very difficult and expensive to find a tach that fits the 3 cylinder rocket.
This is what some of us here use, and it works really well and gives you RPM, water temp, engine vaccum, voltage, and a bunch of other stuff directly to your GPS. https://buy.garmin.com/shop/alt-image.do?pID=38354&img=ImageScreen3Large
Odd, isn't it? I guess they figure you wouldn't be able to see the forth digit anyway with a full readout but x100 would be easier to read and make more sense than x10.