The trio, who were arrested on May 29, had been sent to jail custody yesterday after their five-day police remanded ended, IGP Central Western Range L R Bishnoi told PTI today.
The police did not apply for fresh custody as their interrogation and investigations had been completed.
As the foreigners refused to eat food provided by the jail authorities, the prison doctor referred them to the Gauhati Medical College Hospital yesterday itself, he said.
The Russians, identified as Denis Rozkov (43), Alexei Muratov (35) and Andrei Kilin (45), are here on tourist visas.
The three other persons arrested are Dinesh Kotian (44) and Dinish Balan (40) of Mumbai and local Nabajit Das (40).
The foreigners would be in judicial custody till they apply for bail, the IGP said.
The six persons were arrested here after a complaint was lodged that they were illegally collecting money from the people and were giving motivational lectures to lure prospective customers to invest through their website.
Interrogation revealed that they were fraudulently collecting money through their website 'www.mmmindia.in' promising people with spectacular returns on their 'donations' for charitable work, Bishnoi said.
The men were arrested under Section 4/5 of Prize Chits and Money Circulation Schemes (Banning) Act, 1978 that prohibits running or participation in illegal money circulation schemes, and also booked under various sections of the IPC.