Officials say they believe there had been little contact with the public and that the lifeguards spent most of their time in the lifeguard stands.
"We immediately executed our COVID response protocol and contacted the local health department. All lifeguards were notified of the exposure and will be tested within the next 24 hours," said Rehoboth Police Chief Keith Banks.
The three people are asymptomatic and currently quarantined at home.
This comes as health officials urge people who recently went to Dewey and Rehoboth beaches to get tested because of another coronavirus cluster.
Delaware warns those at beaches during senior week should be tested
The Starboard Restaurant in Dewey Beach was converted to a community COVID-19 testing site on Monday.
And there will be more testing available Thursday morning at Epworth United Methodist Church in Rehoboth.