No 6 AWG Cable

I'm doing a basement remodel and I just uncovered a spot where the previous owner ran individual no 6 awg cables for the feed to an electric range. There are 3 black no 6 awg cables and a green one. The cables are run through holes in the floor joists... about a 50 foot run. Is there any real issue with that? I'm used to seeing the 3-1 individual wires wrapped into one cable, not run individually. Is it a big deal? I could probably put it in conduit, but it would be a real pain in the ...

Single wires must be in conduit
I would not leave them like they are

Probably MUCH easier to just replace the whole run with cable.

YES. Please do this ASAP!

When I rewired my range I had a lot of trouble finding 6-3 ROMEX® wire in the Big Box Stores, although I did manage to find it in the end. Look carefully.
That may be why you have what you have.

6-3 ROMEX® is expensive and very heavy to pull, so if you were to replace what you have, then keep this in mind. It is a workout and from that perspective alone probably well worth every penny getting a pro to do.

I needed to replace an old range cable that was only 8-2 and wanted to upgrade to a four wire range receptacle.

I chose 6-3 ROMEX® because that meant not using conduit and I could also staple the ROMEX® along the joist since my basement is unfinished.

334.14(C) In Unfinished Basements. Where the cable is run at angles with joists in unfinished basements, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards.

Remove the current installation and replace with 6/3 NM cable. Ian hit all of the important points. Listen to him.

hmmm. I expected more of a split decision. I guess I'll put in conduit.... it's a cheaper option since I have some on hand already. Just out of curiosity, what's the real danger in the way it was installed? ....Less protection from penetrations? Oh, and is the plastic conduit OK?

We are not here to tell you what you want to hear. Make sure you size the conduit properly. I have a feeling it would be easier to run new NM cable.

Nor sure if its penetration - IE insulation is thinner then a cable
But I've never seen single wires run anywhere without conduit
Except as a Temp Christmas display extension cord

There's really no split decision on this with anyone that knows basic wiring conduit VS cable

Yup, grey "plastic" conduit for electric wire