MSO (Monadic second-order logic) Logic On Words Let L be a language over $\Sigma = \{a,b,c\}$ that contains all words, where the length $|w|_b$ (number of all b's) has remainder 1 if divided by 3.

MSO logic over words are definded as follow:
 

I want to express this language using MSO logic. But my problem is how can I count the b's to calculate the reminder? So it would be easiert if I just look on $|w|$, cause I could use max.
Start with the following: There are three disjoint sets $X_1,X_2,X_0$ that together hold the $b$-positions of the string. The last $b$ is in set $X_1$,