6/3 NMB in non-metallic conduit?
Don't make your life harder than it needs to be!

First off, shucking NM of its jacket is no good, because the wires inside NM cable aren't marked properly for individual use.  Second, pulling NM through outdoor conduit is unacceptable because NM is no good in wet areas due to the paper separator layer between the jacket and the wires (which wicks moisture up the cable, leading to hard-to-troubleshoot failures later), and all outdoor conduit is considered a wet location by NEC 300.9:


  300.9 Raceways in Wet Locations Abovegrade. Where raceways are installed in wet locations abovegrade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations abovegrade shall comply with 310.10(C).


Third, there's absolutely no reason to make your life this hard: pulling 4 6AWG THHN wires through your conduit (hot, hot, neutral, ground) will be far easier than trying to jam a cable through there.  So, I would simply get an appropriate amount of 6AWG THHN (black, white, green is fine) and use that instead.