The incident took place late last evening when the three girls and a boy went to get vegetables from a field located near the lake under Pardi municipality limits, said Pardi police sub-inspector B J Sarvaiya.
One of the girls, Jhalak Patel (11), slipped and fell into the lake while she was trying to wash her feet.
Seeing Jhalak drown, her friends - Khushbu Patel (9) and Roshni Halpati (9) - tried to rescue her but they too slipped into deep water, the officer said.
By the mid night the three were taken out and taken to hospital.
"...But were declared dead (at the hospital). The Pardi municipality had recently deepened the lake to 20-feet but had not put any signboard about it. We have told municipality to immediately put a signboard," Sarvaiya said.