How about 65mm, 90mm, and 127mm? These lenses tend to be relatively common and inexpensive, and they are a nice nearly-geometric progression of focal lengths.
An alternative would be 50mm, 90mm, and 180mm. Each step in this progression is approximately double in focal length.
I recently bought an RB67 pro S camera, and I have collected a lot of lenses: 50mm, 65mm, 90mm, 127mm, 180mm, 250mm, and 360mm. There aren't many situations that would be out of range of this collection, and the steps between the lenses are relatively fine, so one could get nice framing with relatively little cropping of the image, especially if one were to "zoom with the feet" a little bit.
The RB lenses are relatively inexpensive. I suspect the same would be true for the RZ lenses, though perhaps not quite as much of a bargain as the RB lenses.