Hi, I've been told I'm driving with 3 clyinders, instead of 4, also told it's a headgasket. The headgasket has been replaced twice,car has 40 thousand miles,1998 Hyundai Elantra. Car feels like it's missing,at a stop, wheel vibrates. Engine light is on constant. It's been parked for a month,except for going a short distance a few times,losing some water from radiator,just not quite half a glass. What could it be?

It sounds like the leaking head gasket is allowing coolant to leak into the cylinders. Have you noticed white smoke from the exhaust? Coolant could also be leaking into the oil. You should check the oil dip stick and see if this could be happening. Attaching a scan tool to the data link connector, under the dash, will allow you to know what codes are being thrown, since your Check Engine Light is on. You really need to assess if it's worth sinking more money into your car.

It's very unusual for a such a young car to require replacing the head gasket twice. Do you know why? Was it standard overheating?

Blown head gasket is tested by removing spark plugs, insert compression tester, crank engine 4 or 5 revolutions take reading of each cylinder low reading may be gasket, valves, or rings. Has compression test been done, immediately following gasket installation.
If you put oil in cylinder and retest, may reveal ring problem.

Sounds too early for a head gasket. Has anyone checked the plugs of wires? A bad plug or wire will kill a cylinder.

Some coolant usage may be normal (slow evpaoration, etc.) What you're seeing doesn't sound excessive, yet.

The check engine light: Misfire code (related to ignition?) Could be other sensors out of range. I think I'd get another opinion before I'd authorize majos repairs.

Get the code(s) checked and let us know. Good luck!