Karl is right. It’s their way of indicating a triplet! Borrowing from the western method of writing a triplet. A triplet simply means (as Karl mentioned) 3 notes in the time of 2 notes.
A cliche way to think of it is to count 1&2&3&4&, 1&2&3&4& etc.
Next keeping the exact SAME tempo in the place of 1&, count instead 123. This transforms the pattern into…
123, 123, 123, 123 or more specifically
123, 223, 323, 423 etc.
However Yosare bushi is of course in a three meter to begin with so it would be more like…
123,123,123.
Or 123, 223, 323 (to be more specific )
So the way a triplet will superimpose itself over a meter in three will be to interact between the even beats.
Like… 123, 12, 12 for example.
( in this case the first beat is counted in three and the other two beats in two Yet the overall rhythm uses the pulse of three)