There is no issue running an unbalanced load through the breaker.
I would probably make the adapter, since code inspectors have no business looking at what's plugged in to an outlet. They should be concerned only with what is part of the fixed structure. You can always unplug it when they come around.
But that kind of outlet makes me think it's a business with a compliance department and lawyers. Even if the inspector doesn't have a right to complain about your bootleg adapter, your insurance company will if there's ever a fire, related or not. And personally, I don't like shared neutral-ground conductors.
What's going to really get you though is putting a 20A outlet on a 30A breaker. If your AC failed in a way that caused it to draw 30 amps, it could overheat that outlet and start a fire in the wall.
So. You're going to want to change the breaker, and when you do, I would recommend you rewire as follows:
1) Use the existing combined neutral-ground conductor for ground
2) Pick one of the phase conductors, and clearly mark both ends with white tape. Use that for neutral
3) Pick another of the phase conductors and cap it with wire-nuts on both ends
4) Take the last phase conductor and put it on a new, 20A single pole breaker. Install a 20A outlet for the AC unit.
For extra credit, take advantage of the wiring in the wall to bring TWO 20A circuits to the box where the AC will plug in. Steps 1&2 as above, then.
3) Buy a double-pole breaker. Be advised. In a 3 phase panel you need to particularly select a 120/208 double-pole breaker, as opposed to just a 208 double-pole breaker.
4) Wire the two remaining phase wires to the double pole breaker.
5) At the outlet box, install two seperate 20A outlets. One on each phase, sharing neutral and ground between them. If it's a single gang box, break the tab between the two hots.
This will let you dedicate one 20A circuit just for the AC and still have plenty of power on hand for other uses.
NEMA 14 is the universal donor
Your NEMA 14-30 connector is the type you want. It has
- neutral (white or gray, 120V to L1 or L2),
- Safety Ground (green, green/ehite or bare, near 0V to neutral, near 120V to a hot)
- both hots (any color yet unnamed, same color ok, black and red commonly, L1 and L2, 240V across them)
What voltage is this thing?
You need to look at the machine's nameplate and see what it actually is. What's more, get the manufacturer's instructions for the unit and see what it says. It is illegal to connect it contrary to the manufacturer instructions, and that makes it illegal to fail to read them.
US-market large heaters (2000W and up) are made for 240 volts, needing to connect to L1 and L2. They do not use neutral. Many smaller heaters made for permanent installation are also 240V.
US-market small heaters are often 120V. If they are 120V, they have a hot (either L1 or L2, never both) and neutral.
European and rest-of-world heaters are all 230/240V, because everything there is 230/240V. Most will tolerate 240V because they're built to be compatible with the UK and some Asian nations which are 240V rather than 230V. Unlike us, they have 240V between hot and neutral. They use brown for hot, and blue for neutral. First, you must check the instructions or with the manufacturer as to whether they allow North American style hot-hot connections. If they do, connect brown to L1 and blue to L2, or vice versa.
You also connect a ground wire, always. Doing it bare 2-wire is not allowed, unless the heater is listed as double-insulated, which it will say rather directly if it does, because they had to pay a lot of money to have it certified as such. Don't assume this.
Cord-and-plug connected, or hard-wired
You can only go cord-and-plug if it makes sense for the thing to be portable. But then, you can fab it yourself. You need to visit a real electrical supply house and obtain proper cordage, which is special electrical cable made to be a cord. The size of the required wire depends on the rating of the heater. Then you need proper strain relief on the appliance end, and a good fitting plug on the wall end. Getting right stuff that matches is the reason to go to a proper electrical supply; they can do it; big-box can't.
If it's hard-wired, you will need to go to your town and pull a permit, then have it inspected when you're done. If you're not super good at this (and you aren't yet), the second one may be the way to go, because a set of smart eyes will look it over for you. On the other hand, if you're doing something you should not, he can shut the whole thing down. Also in most jurisdictions you must hire an electrician for units you are renting out, or if you have been banned for being really bad at it.
If you hardwire it, you need to run 10 AWG wire, since your circuit breaker is 30A.
Best Answer
The problem with doing this is you may have, say, a 30 amp double pole breaker on your 240V line which may be 10g wire, and the you come off it on one phase with a 120V circuit with 14g wire, the 30 amp breaker is to high for 14g wire, which then is not protected from melting.
The US has moved to 4 wire 240V service, to accommodate appliances that have internal 120V electronics.