Three hippopotamuses were found dead at the Karlsruhe Zoo early today. Authorities said an elephant did it.

The hippopotamuses - two adults and a new-born calf - suffocated from heat and steam after water hotter than 140 degrees flowed into their pool, zoo officials said.

The water in the pool is normally kept at 68 degrees.

Puzzled by the circumstances, the zoo called in detectives. After an investigation, they concluded that a three-and-a-half-ton Indian elephant in an adjacent enclosure had pulled free of its chain, reached over security fencing and opened a hot water valve with its trunk.

A zoo spokesman said the 30- year-old elephant, a female named Rani, ''has been perfectly behaved all her 29 years here.''