My recommendation would be to have the cable re-pulled, its the only safe long term option.
You can run DMX512 over just about any form of cabling when push comes to shove (combat engineering and all that) but its a really poor place to start from in an installation. It might be all right, or it might work 98% of the time, you just don't know. Its the twisted pair-yness that provides DMX512 cable (and balanced audio and loads of other signal types) the ability to withstand interference from other cables, particularly power cables, and three core isn't known for it's twists.