The third spot is at the top of the heater, there are several connections from the piping & fittings that attach to the heater where leaks can occur from the washers inside the flex connectors that have gone bad.
Once you have determined where the leak is coming from, then shut off valve off that is typically right above the water heater, either a ball valve type with a lever handle or a gate type valve that has a round handle that you turn, in this case you follow the lefty loosey, righty tighty rule. If its a lever handle, you simply push it up or down, depending on the position of the valve.You also want to shut the gas off, until repairs are met.
If you cannot find or are able to shut the valve off at the water heater,then shut your main off to the entire house or call your local plumber immediately.Its always a good idea to have a water alarm or a pan under the heater to protect the inside of your home from getting damaged. We just replaced a water heater in a hallway that leaked and caused an entire office room to flood because there was no pan or anything to capture the water.So make sure before replacing that water heater, that you consider all your options to help prevent any future leaks damaging your property.
We sure hope this blog helped in answering your questions on what to do on a leaky water heater. As always, thank you & please reach out to us anytime here at the Twin Plumbers!