NM cables are not allowed to be in conduit, as such. That is not a 'listed use'. You are limited to the use of conduit for 'nipples' under 24". You can use conduit for mechanical protection, but you have to derate if over 24". Curiously, if you use a "cable shield":
then you don't need to derate. Go figure...
I'd say that 6/3 CU will go though 2 - 90* and 1 - 45* in the 1-1/2" range, if the derate for your install allows that. Feed from the end that has the LEAST amount of bends and pull from the end that has the MOST bends. Make up the pull-head as small as possible, tape smoothly and use plenty of lubricant. Have someone push the cable while you pull.
I faced this same problem early in my career, and an AHJ told me that I could strip the NM jacket off and use the single conductors IF the individual conductors were marked (the same as single conductors). Some are and some aren't...YMMV.--
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