It's me again.

I checked up on the consumption of too much water.
It's called "hyponatremia." As a person drinks large amounts of H2O, blood plasma increases. As this takes place, the salt content of the blood is diluted. If sweating, you lose even more salt. Consequently, the amount of salt available to the body tissues decreases over time to a point where the loss interferes with brain, heart, and muscle function.
The symptoms generally mirror those of dehydration (apathy, confusion, nausea, and fatigue), although some people show no symptoms at all. If untreated, hypoatremia can lead to coma and even death.

Interesting Footnote:
Water intoxication has become one of the most common causes of serious heat illnesses in the Grand Canyon. Some people hiking the canyon drink large amounts of water and do not eat enough food to provide for electrolyte (salt, potassium) replacement and energy.

I hope the 3 "gallons" was a typo and you meant 3 "quarts."
If your DH has truly been drinking 1-3 gallons a day for an extended period of time, you should call your doctor right away!