A sub-panel has a cable of three 12 gauge wires for HVAC coming into the box through a big knock out hole. The HVAC people did not put a connector on the cable. Is there a connector designed to be added after the fact which would snap around the wires and get pushed into the hole from the inside of the panel? The panel is in a wall with no good way to get to the outside of the box at this point.
Yes, there are connectors that can be slipped onto the cable and popped into place in the panel knockout. They do not snap over the cable though, you must disconnect the wires and slide the connector over them.
This certainly sounds like a code violation and will require ripping the wall open to repair; the cost of that should be borne by the company that did the work incorrectly, but it may be difficult to get them to actually pay for it, unless you can work via a licensing board (ie, if they have an electrical license allowing them to do electrical work in connection with HVAC work, you should be able to take this up with the board that issued that license, which might cause them to pay for the rework needed to tear the wall open, fix this, and repair the wall again.)
If they are just unlicensed yahoos, I'm not sure what you can successfully threaten them with other than an exposé in the local news media.
If you have 3 romex cables, those need to come into the box through 3 separate openings with clamps - I don't believe there's any such thing as a clamp listed for 3 cables in one hole.
If you have 3 THHN (or related) wires, they should be in a conduit to get to the panel, and that conduit should be connected to the panel box.
In either case you need to disconnect the wires, take them out of the box, put in the proper fittings, and put the wires back into the box. If you need to tear a hole in the wall to do that, you need to tear a hole in the wall to do that.