The ACB sleuths interrogated the three accused men at its headquarters for the third consecutive day.
They were first taken to government-run Osmania Hospital for a medical check-up.
Revanth Reddy, a Telugu Desam Party member of the Telangana state legislative assembly, was arrested by the ACB on May 31 when he was offering Rs.50 lakh to nominated member Stephenson to make him vote for the TDP-BJP candidate in the elections to the Telangana legislative council.
The ACB, which had laid a trap on a complaint by Stephenson, also arrested Revanth Reddy's aides Sebastian Harry and Uday Simha.
The entire episode was recorded on a spy camera installed in Stephenson's house.
The next day a city court sent them to judicial custody for 14 days. The court later granted the ACB four-day custody of the accused men.
The ACB is questioning them to find out the source of Rs.50 lakh and also from where they planned to bring another Rs.4.5 crore under the Rs.5 crore deal.
They were also being questioned about the complicity of other political leaders.
The interrogation on the third day assumed significance in the wake of release of an audio tape - a purported telephonic conversation between Andhra Pradesh Chief Minister N. Chandrababu Naidu and Stephenson.
The audio tape was aired on Sunday night on T News, a channel owned by Telangana Rashtra Samithi (TRS).
Describing Naidu as the mastermind, the TRS leaders have been demanding that he should be included in the case as the prime accused.
Meanwhile, a city court adjourned to Wednesday the hearing on Revanth Reddy's bail petition as the ACB sought more time to file the counter.