Two Olympic bronze medal winners, Kyrgyzstan weightlifter Izzat Artykov and Moldovan canoeist Serghei Tarnovschi, have failed doping tests.
And Indian wrestler Narsingh Yadav is also out of the Rio Games and banned for four years as the World Anti-Doping Agency (WADA) won an appeal at the Court of Arbitration for Sport (CAS) on Thursday.
Artykov has had his Olympic bronze medal from the men's 69-kilogram class revoked after testing positive for strychnine, the CAS said.
Tarnovschi, who won bronze in the single canoe 1,000-metres, has been suspended from the double canoe 1,000m "following adverse results from a pre-competition test," the International Canoe Federation said.
He finished behind Germany's Sebastian Brendel and Brazilian Isaquias Quieroz dos Santos. Should he eventually be disqualified from the race, Russia's Ilia Shtokalov would move into the bronze medal position.
Weightlifter Artykov, 22, entered the Rio Games as the 2016 Asian champion but finished third behind China's Shi Zhiyong and Turkey's Daniyar Ismayilov.
Colombia's Luis Javier Mosquera Lozano will move into the bronze medal position having finished fourth in the standings.
CAS meanwhile confirmed that Brazilian cyclist Kleber da Silva Ramos and Chinese swimmer Chen Xinyi are both disqualified from their Olympic events after accepting suspensions following positive doping tests.
Chen had been suspended on August 12 and Kleber on August 13.
Chen, 18, failed a doping test in Rio on August 7, the Chinese Swimming Association said last week. She had already secured fourth place in the 100-metre butterfly before testing positive for the banned diuretic medication hydrochlorothiazide. She was prevented from entering the 50m freestyle.
Kleber, 30, tested positive for the blood-booster Cera shortly before the Games, global cycling body UCI said last week. He did not finish the Olympic road race.
The Indian National Anti-Doping Agency cleared Yadav, a former Commonwealth Games champion, on August 1 after a positive test for the anabolic steroid methandienone from July 26. Both samples of the doping test came up positive.
However, WADA was successful in its appeal to CAS and though a first-time offender, Yadav has been given a lengthy and immediate ban.
He is now unable to take part in the 74kg freestyle category at the Games on Friday.
A handful of doping cases have been announced at the Rio Games, mostly from tests carried out in out-of-competiton testing before the Olympics.
Weightlifting has been overshadowed by several doping scandals in the run-up to the Games with Bulgaria banned from qualifying after 11 athletes were found doping in 2015.
Other countries, including Kazakhstan, Russia and Belarus, have had multiple athletes return positive tests in recent years.
Strychnine, found in Artykov's sample, is a potentially fatal poison but it has history in the Olympic Games having been legally used to help American Tom Hicks to marathon gold in 1904. Supporters plied Hicks with strychnine and brandy to get him through the race, though he required medical treatment upon finishing.