The orders, approved by King Salman and published by state media, also brought forward the start of curfew in the three cities to 3pm from 7pm from Thursday.
Saudi Arabia introduced the 21-day curfew on Monday after registering a jump in infections. It reported its first death on Wednesday, in Medina, and 205 new cases taking its total to 767.
Across the six-nation Gulf Cooperation Council the tally rose to over 2,250 with six deaths, as Oman registered 15 new infections and Kuwait four.
Saudi Arabia has taken rigorous steps to contain the outbreak, also including halting international flights, suspending the Umrah year-round pilgrimage and closing mosques, schools, malls and restaurants.
The curfew and other restrictions have altered the rhythm of daily life in the country of some 30 million, many of whom enjoy late-night gatherings at coffee shops or private homes.
Nasif Erisik, a Turkish resident who plays cards most nights with friends at one of their homes, said the group has resorted to online gaming to keep in touch.
"Corona has ...changed our habits and everything in our lives," he told Reuters.
The authorities have said curfew-breakers would be fined 10,000 riyals ($2,665) and repeat offenders could be jailed for up to 20 days. Those circulating videos of violations or encouraging people to do so could face up to five years in jail and a 3 million riyal ($800,000) fine, the public prosecutor said.
Authorities in United Arab Emirates, the region's business hub, have urged people to stay at home but not announced an official curfew or suspended work.
On Wednesday, they directed food retail outlets to remain open 24 hours a day but not exceed 30% capacity to ensure physical distance of 2 meters (6 ft) between shoppers.
Organizers of the Expo 2020 Dubai world fair, scheduled to start in October and estimated to draw 11 million overseas visitors, confirmed one coronavirus case among staff and said they were reviewing plans and preparations.