Odd smell from hot water if I don't use it for a while, what could it be?
The bad smell is not dangerous. It is usually caused by 1 of 2 things. The anode in the water heater has dissolved and there is a bunch of gunk in the bottom of the water heater that needs to be flushed out using the bottom drain on the water heater. The second reason is trapped air in the tank, I have found this more often on systems with pressure tanks that the bladder has failed, air in contact with the water makes it stink. At 3 years it would be a good idea to pull the anode and make sure it is still intact. Regular anode replacements and flushing (2-4 years depending on water quality) can double or triple the tank's life.