Those killed were serving as military instructors within a unit of the Syrian forces.
Doctors are fighting to save the life of the third solider who is in hospital.
The ministry said that the Russian military instructors were embedded with a Syrian army unit and – along with a Russian military adviser – had come under mortar fire from a group of militants.
Russian President Vladimir Putin
The deaths come amid growing tensions between Russia and the US over Syria, which escalated sharply after US President Donald Trump ordered a missile strike on a Syrian airbase after it was claimed the Syrian leader Bashir al-Assad, backed by Russia, launched a chemical attack on his own people.
Earlier today the US Secretary of State Rex Tillerson told reporters at a G7 meeting that he hoped Russia would end its support of the Syrian leader.
Mr Tillerson said: "It is clear to us the reign of the Assad family is coming to an end.
Horrific images depict sickening chemical gas attack in Syria
Tue, April 4, 2017
At least 58 people, including nine children, were killed in an air raid that released "toxic gas" on the rebel-held Syrian town of Khan Sheikhoun
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A child receiving treatment at a field hospital after an alleged chemical attack in Idlib, northen Syria
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US Secretary of State Rex Tillerson at the G7 summit
“We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in (Syrian President) Bashar al-Assad.”
Most countries in the West have blamed Mr al-Assad for the gas attack last week which killed 87 people.
Mr Trump responded by firing 59 cruise missiles at a Syrian airbase.
Smoke rises after a bombing raid in Syria
The actions have caused increased hostilities between the old Cold War enemies as Russian President Vladimir Putin has stood by his Syrian ally, who has denied being responsible for the gas attack.
The Russian defence ministry has reported a total of 29 Russian servicemen killed in Syria since September 2015, when Moscow was invited by Damascus to assist in the counter-terrorism campaign. One more fatality was a suicide.
More to follow…