CRPF Constable Ajit Singh fired shot dead the three colleagues inside a camp in Battal Ballian area on Wednesday night after a heated exchange of words, a police officer said.
Devon Erickson, 18, and Alec McKinney, 16, who was listed on the court docket by the name Maya Elizabeth McKinney but who identifies as male, were both arrested on suspicion of a single count.
Thousands of protesters have massed at the army complex since Saturday, urging the military to back them in demanding the resignation of President Omar Al Bashir and the formation of a transitional government.
Police Chief Hank Stawinski said at a press conference that around 12 people were at a 2-year-old's birthday party when an individual fired shots into the gathering.
"The Presidential Guard retaliated, preventing this group of soldiers from approaching the presidential palace," said the source.
Mya Thwate Thwate Khaing, who had just turned 20, had been on life support since being taken to hospital on Feb.9, after she was hit by what doctors said was a live bullet at a protest in the capital, Naypyitaw.
The military operational heads of the two countries spoke over telephone and agreed to discuss each other's concerns that could disturb peace and lead to violence in the Himalayan region, a joint statement said.
The emergency services counted 44 injured at 4am (0200 GMT), some brought to hospital by ambulance and others who made their own way there.
It was the worst single attack in years in a restive region where a long-running Muslim insurgency has killed thousands of people in a fight against central government rule in overwhelmingly Buddhist Thailand.