Hey, welcome! As for your question, are you trying to do 6C right after 3C, because that will not combo unless 3C is a counter hit starter. Instead, go into 236D or 214D~A.
If you're talking stuff like about 214D~C (zaneiga/devouring fang), it takes some timing. If you're hitting it low, you have to do the 6C as soon as possible. If you hit it while the opponent's higher up, you'll need to delay it slightly for it to hit.
As for the dash part... If you're right by the opponent when you hit 214D~C, you should do a regular 6C w/o a dash. If you hit them with 214D~C and you're about a person length away, you'll probably need to add a dash to have it land.
Out of curiosity, you are on Continuum Shift Extend?
EDIT: Beat to it lol.