None of the three returned calls seeking comment on Monday.
The three secretaries of state -- for tax affairs, industry and internationalization -- had accepted trips and tickets to the Euro 2016 soccer championship in France that were paid for by energy company Galp.
They have since paid back the costs of the trips, which were first reported by Portuguese media in August 2016. The three have denied any wrongdoing or conflict of interest in accepting the invitations.
Galp, which sponsored the Portuguese team at Euro 2016, has also said there was nothing unethical about its invitations.
The government earlier said that Rocha Andrade, Vasconcelos and Costa Oliveira had requested they be named as suspects when they resigned, so as to better defend themselves. The prosecutor's office did not say they had volunteered to be "arguidos".
The case is another embarrassment for Prime Minister Antonio Costa, who is already under pressure from the opposition to sack his interior and defense ministers over deadly forest fires in June and a major arms theft from a military depot.
Some analysts say a wider government reshuffle may follow, although those two cabinet ministers are expected to stay on after Costa's Socialist minority government defended their actions and general competence.
Last week, prosecutors said international arms dealers and militants were suspected to be behind the theft of more than 100 grenades and ammunition from a poorly-guarded military arms depot at the end of June.
The attention-grabbing theft has piled up pressure on the government after it was criticized for the handling of the country's deadliest-ever forest fire on June 17-18. The blaze killed 64 people.
The minority government has seen its approval ratings grow thanks to strengthening economic growth and improving public finances. A survey on Friday by pollster Eurobarometro put the Socialists at 40.4 percent of voting intentions, well ahead of the main opposition Social Democrats, on 28.6 percent.
More about: #Ronaldo
