You don't just skip the first 20 rows of chart B, you just start it on a different row. My thought would be that you work Chart B from 21-40 while you're working Charts A and C for rows 1-20 two times
Work Charts A and C from rows 1 to 20 and repeat 1-20
and at the same time you work Cart B from 21-40 and then 1-20
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry