Is it acceptable to run insulated 14ga single strand through studs/
joists?

I need to extend the wiring of a 3 way switch. I have 14/2 "romex"
and 14ga single strand on hand. If it is acceptable (not against
code), I'd like to use the materials I have instead of buying a $100
roll of 14/3 to go 15'. I plan to make the extension connections in a
j-box and run the 14/2 + single strand through ceiling joists.

Thanks,
-TJ